I reckon you are lucky you didn`t end up with that van, at that mileage you need to budget another £2k to keep it on the road another 12mths,for example,new gearbox(recon),dual mass flywheel,clutch asssembly etc.etc,,high mileage vans slowly need every bit replacing bit by bit,i am talking through experience.
I`m tempted to offer you my set up as i would like a larger van and a larger water tank,i know what you mean about getting the kit first,its what i have done but i got everything at a decent price and its probably worth what i paid for it. However,although i am well chuffed with a 57 plate combo 1.3cdti,18k miles and brodex 250 litre tank etc(got the brodex gear unused but 2nd hand), i would like a scudo/expert/dispatch with a larger tank. So with your budget i recommend you look for a van this size that can carry a 500 or 650 litre tank,circa £3500 should get you a 56 plate in great nick with service history and sensible mileage. £550 will set up a nice HF5 RO system inc all connectors. Gardiners as mentioned for a user friendly pole,Look for a 2nd hand tank and system,you have some money now so dont skimp on the RO system,the HF5 i put together is the bit of my gear that really excels.