As a converted Transporter owner, I would stay away from them if you want value for money. To sleep 4 and carry bikes you’ll be looking at least a long wheel base medium size Van or a short wheel base with a pop top. I would look at converted vivaro’s for value for money. £10k on transporter will only get you a decent T4 model, not a T5.
Cheapest option is to buy a panel van and convert it yourself, that way you may get a decent age/condition T5, Converting yourselfcan cost between £1500 and £5000. Depending on whether you want full 12v and 240v, fridge, cooker, water, pop top , rock and roll bed etc. Have a look on http://www.vwt4forum.co.uk for some inspiration.