How many people?
Fixed beds are ok if there's two of you or you have grown-up kids with the same bedtime. Otherwise you'll have to make up the dinette, put kids to bed and then either sit outside or lie in bed until your own bedtime.
Vans are basically all the same (at the lower end) but some makes are better than others. The ones from 20 years ago that haven't gone bust are a good bet - like Bailey for example. You need to consider the layout as much as anything else. There are only so many ways to lay a caravan out, you have the option of fixed bunks at one end for kids, a dinette at each end, or an end and side dinette. It's nice to have two dinettes or a spare table at least with kids, because they can play on one whilst you sit in the other location.
Older single axle vans tend to be light enough to be towed with a normal car, Passat etc or Golf sized. There's lots of info online about towing weights.
With regards price - I would say a liveable van could be had for £1500 from ebay or caravantalk etc classifieds. Although there will be some shockers for that price, there will also be some sound vans that are in good shape. Smell when you walk in is a first clue, then tap the walls and floor all around the van and especially right in the corners - which will be under seats, in cupboards etc. Three grand seems to be the next price point, you can get some nice stuff at that price.
Check tyres too - van tyres never wear the tread out no matter how old they get - they just perish and ultimately explode. Do not buy anything with cracked rubber on the tyres, this is very common. There's a way of reading the manufacture date from tyres, google it - they must be no more than 5-7 years old. About £200 for three tyres fitted.
Specs of 20 year old vans are all broadly similar. Fridges and cookers have been around for decades of course, but yoiu should be getting double glazing, a shower and a cassette toilet too, and a heater. Blown air heating is a nice touch. Also blinds and flyscreens are nice, but they do tend to perish break after 20 years so check they work. Unless you are either infirm or need to park it backwards up a slope at home (like we do) you don't need a motor mover. Caravans are very easy to move by hand on the flat, even with two people. Awnings are often thrown in, they can be worth £150 and are useful.
Check the van is level, suspension can collapse on one side (or both!) Also check the hitch damper. It should be almost impossible to push or pull the towing hitch in and out, it's got a grease filled damper. These can fail or leak. They are cheap and easy to replace though and they can be towed wth a bad one but it's rather scary and a bit uncomfortable. If you're not going far then it's not a big problem, as long as you do fix it asap.
When buying, it's best to look for vans that are parked up at someone's house and have been holidayed in. There are quite a few people who buy and sell cheap old vans from a unit or a field - some are doing a good repair job and are reputable, some are dodgy geezers flogging moldy shite.
Everyone says they don't use the shower, I use mine a lot. It's much nicer than some site showers, there's no queue, no-one else's pubes all over the floor and you don't have to try and get dressed and dry in a dirty puddle, which always seems to be the issue with site showers. It's also brilliant at bike races!
If you're staying a weekend, don't bother with an awning. Entirely optional and saves a lot of time.
whether theres anything 'cool' out there rather than your bog standard caravan that looks like all the others.
Tab teardrop vans are cool, probably hard to find second hand though!
it's the top bunk ones that fold up onto the wall during the day that aren't really up to prolonged use.
Not convinced by that. They are rated for use for kids, not sure they would get away with making shoddy stuff for that purpose.