Most of these checks are common sense and 95% of 20 year old and over dandys wont have any of them, they where built to last and they do
1 Chassis do corrode so get underneath and poke around.if its corroded any where its not the end of the world so long as its safe to tow and you know some one who will do the work, it is a very simple chassis design, include this in any offer you make.
2 Suspension does loose its bounce and sag a bit with age(don't we all)
There is no scientific answer as to how much clearance there should be between the tyre top and the wheel arch opinions vary. Braked suspension units are now cheaper than ever and you can normally get a new set with brakes included latest price i have seen is just over £200 a set on ebay,you may be tempted because of the light weight of the unit to fit even cheaper none braked suspension but be careful the unbraked legal trailer weight is low and it is a lot lower for individual cars,best to check on this.
3 Wheel bearings do the usual checks spin the wheel look for smooth quiet rotation and grasp the tyre on each side and try to wobble it,wheel bearings are not that expensive any way.
4 Tyres check condition of the walls and how much tread, the ply rating marked on it is important, my unit is max load 750kg and i need 8ply if you replace them get trailer tyres not car tyres, average price about £40 each (quote the max loaded weight to the tyre co and they will know what tyres you need) there is also a manufacturing date on the side wall,due to the degradation of rubber its possibly best not to use tyres over 5 years old.
5 Brakes most if not all dandys you are likely to come across have over run brakes meaning when the car brakes the dandy will, difficult to check without towing but you can check the parking brake works without it pulling up full travel
6 After that its checks of the hitch coupling and look all round the trailer for damage and make sure the lights work.The hitch is connected to a damper with the handbrake off you should be able to with slight resistance push the coupling completely in by hand,it should return all the way slowly,if not it may be faulty.
7 Now we come to the tent itself in some ways its better if the outside is filthy because they really clean up well and you may be able to knock the price down, things to check for are slits and damage on the out side, if present it could be a case of no thank you, patches on the outside are not wanted. The inside is a different story sometimes you will find small slits or nicks on the inside (these can happen when erecting or taking down) if its only the inner layer then no real problem they are easy to repair and don't effect the water proof aspect(but still try to knock the price down) the pvc is guaranteed for 5 years what a joke it will still be waterproof in 35 years.When you see it being erected take a mental note of all areas that hinge and check the condition of these i know
i keep saying this but no real problems again these units are built to last and can be maintained with a simple tool kit and pop rivet gun.
Check the base boards under the bed they can split but again that phrase no real problem and are easy to replace its only thin ply,the load is taken by metal cross bars.The internal checks would be the same for all makes of campers,gas and mains electric if fitted should be checked by a professional,upholstery can be cleaned or recovered.
One final note there should be a number stamped on the a frame the last two digits are the year of manufacture
ie****89 would be 1989
Just my way of looking at it i know but condition to me is more important than age my Dandy six is 26 years old and has years left in it
One important omission the floor this can be a weak spot do a little bouncing on it to make sure it isnt soggy,there are a few tips on the forum about curing it and all i can say is it depends on how bad it is and how much you are paying but it doesn't mean a scrap dandy like most things it can be sorted.