I don't own a dark blue Dandy I'm not really up to that sort of spend level. The only people I know who have owned both are Vicky and Nick. See: [You must be registered and logged in to see this link.]
Try searching through post for more specific reasons. Old Lad has a dark blue Dandy and will have observed plenty of other Dandys as for the other way round there are just not as many dark blue units so most haven't done a direct comparison though both Mike and Helen will no doubt have observations. You are no doubt not as wantonly sexist as I am but if you are don't presume Vicky's thoughts will be on the girly level of trim quality, Vicky is an engineering type (mech eng?) My own thoughts gathered through trying to remember what I've heard rather than actually comparing are that I think some of the ply sections were replaced with OSB board [You must be registered and logged in to see this link.]
Try googling OSB Vs Plywood but sit down with a cup of tea and biscuits or even a sandwich first. Though ironically the pannel that would cause you the greatest grief and expense to replace, the floor has always been in OSB (OK not certain on 60's originals)
I notice the light blue ones were lighter in weight than the new ones. Is this because there's more kit in the new ones or possibly a different way of calculating the weight. Such as including battery & gas bottle in the ex works weight or whatever they call it now. Or could it be extra insulation not present in the light blue ones, but now fitted and not included in the standard stated weight, meaning that many light blue ones are actually heavier than quoted.
"Such as battery and gas bottles" Ha Ha Ha Ha Ha Though there is a greater chance that a Dark Blue may have been factory wired (especially if factory fitted with a Propex heater) all that was paid for separately and never included in the tare weight. Dandys are notoriously Spartan as sold. Conway and Pennine got increasingly flash kitchens, bathrooms, water heaters and soft furnishings (to up margins) you just about start to see this on late model Destinys. There was a comment that Dandys were weighed off originally but not for new models so there was weight creep (Ian Smith Riva???) Then again the bed frames went from aluminium to steel there is a change from ply to OSB and look at the comparative weight of the pre and post 1995 kitchen fronts. Weight transfer in opposite direction Glass to perspex windows. Is the weight difference 100Kg? Dunno if I ever empty out my Designer and I'm near a weighbridge I'll check. (You have to have seen my Designer or car to realise how unlikely that is) I'd bet you loose in excess of 70Kg of load though. N.B. If choosing any 500Kg Dandy where the suspension needs replacing Riva see no problem in upping to 750Kg units.
So you are looking for a Designer but are in no hurry. this is a good thing I sense from your comment about the differing methods of measuring weight that you are not deeply immersed in Dandy knowledge yet. Great we can use the time to make sure you get what you want.
My own thoughts. Age isn't important condition and personal preference is. Look at the spread on how old owned Dandys are you won't find that age spread on folding campers (canvas) or caravans. [You must be registered and logged in to see this link.]
What % of caravans that you see are over 40 years old? If somebody's tried they they could have buggered a 6 year old Dandy (newest available) conversely you may find an early 90's Dandy that is comparable with a 2 or three year old folding camper.
There is nothing that can go wrong with them. Provided you have had a good look and the floor is not soft (As in OSB board that got wet not caravan delaminated floor)or the frame rusted away the biggest issue would be a suspension change. If you allied that to a 750Kg suspension upgrade and a spacing beam being inserted (Hitch heights changed) you are still only looking at £600 if the job is done at a garage. There is a constant need to replace rivets or the odd roofing bolt (used to attach roof hoops) but basically we whitter about very little of any real complexity on this site.
Will you be able to tow it? Mike tows a Dandy 6 (lowside Destiny) with a Punto. Now at the winter end of the year he is not too happy with the idea of a long trip. I don't know if that is just him, the worry of heavy winds and snaking or not wanting to do hilly areas in winter. No doubt he will say. If the site i used was correct (and many aren't) a 1200 Punto is good to tow 800Kg [You must be registered and logged in to see this link.]
So you could tow a Lowside Destiny if you wanted. I don't know how stable the side entry Dandys are (I know someone who had a moment with a late Highside Destiny without a stabiliser but they are very different beasts) but the rear entry Dandys are perfectly stable due to their geometry. see [You must be registered and logged in to see this link.]
Though if you have had caravans in the past I suspect you're more bothered about the legality of towing not the practicality. You will know what you feel about any set up.
A lot will depend on how you want to use the Dandy. If saving money is key my first thought is to an older Dandy with a Riviera floor heater as a Propex tends to need EHU as it eats batteries. This may not be the limit it once was as solar cells are so much cheaper so the Propex may be viable on wild sites. I gather a few retirees follow Steam rallies or other summer events for a combination of entertainment and cheap camping.
Dandys aren't the most lightweight to erect so if it's a later model with steel bed frames you may be looking at gas struts but you might well want the gas struts anyway.
The PVC awnings are not light but are bombproof, inserting zip panels in the side helps with the weight but the dark blue Dandys had the option of Acrylic awnings that zipped on. You then need to dry them out but Acrylic dries quickly. Not quite as robust but as always "You Pays Yer Money And........" Also I don't think I'd hold my breath waiting for a Designer with an Acrylic Awning. Not as rare as hen's teeth but I've not seen one yet.
Her indoors. Back to wanton sexism. Dandys are a bit stark inside By the time of the dark blues this was being addressed. The later upholstery looks a lot better. Dandys never needed a roof liner but a roof liner and modesty curtains can improve the aesthetics no end.
One radical suggestion if your used to caravan dimensions and I'm not sure what sized caravan you used to have you thought of a Lowside Destiny? Yes I know you were asking about the newest Dandy possible but I'm here to throw answers and other possibilities back to you.
You don't say where you live but at some point we should be having a rally not too far away Come the caravan is welcome and you can look and as far as I'm concerned when I'm leaving your welcome to tow my Designer although I don't think a great deal will be learned towing till you get the Punto.