We are fairly seasoned caravanners so may be able to give some advice.
In the price range you are looking for you may have to settle for quite an old van.
First of all, check the tyres. Caravan tyres deteriorate quite badly because they are laid up for months on end so be prepared to replace the tyres if they look at all cracked. Try to get a van with a spare wheel too.
Don't be too worried about any scrapes or bashes to the bodywork. Older vans often have minor damage but so long as the van is weatherproof the odd bit of repaired damage is not important.
Facilities wise... I would not have a shower in it. All the sites in the UK have lovely shower blocks and you run the risk of water seepage into the floor of the van. Also, you need to carry all the water to your van, heat it, then carry it all away again. So a shower is a waste of time.
I would however, have a cassette toilet as opposed to a Porta-Potti. The cassette toilet is emptied from outside the van, the porta potti has to be carried through the van and I would be worried about spillages. We restrict our van toilet to wee-wee. Poohs must be performed in the site toilet block!
For cooking; British vans will have a full size cooker. Continental vans like ours only have a hob. I have adapted meals to use the hob only. For instance, you can make toast in a ridged frying pan. So I would say the size of cooker is not that important.
You must have a three way fridge (battery, mains, gas) And make sure your car and van are wired for twin electrics. This means that you will be charging the van's battery as you are driving along and powering the fridge.
Also make sure the van has an external water pump with an Aqua-roll, not an on-board water tank (like ours). The tank is a nuisance to fill.....Aqua-roll much easier.
You will need a gas heater but all vans have those anyway.
For layout choices, I would recommend going to a dealership to look at all the choices available. I wouldn't recommend a fixed bed, (restricts space) or an L-shape (not easy to assemble). Centrally sited kitchen and bathrooms ie, over the axle, make the van more stable for towing but they are not popular because if someone is working in the kitchen, access to the bathroom is restricted.
With a family, the awning is a must but make sure that you get one that has a bedroom annex option. As the children grow, they can sleep in the awning.
I will post again if I think of anything else. I would recommend joining the 'caravan club' or the 'camping and caravan club' Eithert will do and you only need join one as they have similar facilities.
Hope this helps