I would hazard a guess at 4k+ to get the body work sorted out, then all the mechanical, bushes etc so it could be anything up to 4K on top of that (gearbox, VC etc...) Then if you want an engine that can do a sensible speed factor in £2000-£6000 depending if your going AAZ do it yourself or pay for a TDI conversion, then add your purchase price. If you have a specialized conversion like a Westfalia you will easily do another £1000+ on ripped roof canvas, fridge, Ebber, replacing damaged interior parts, vents etc... then there are any of the expensive toys you fancy, and you can go as far as you like with those

I am by no means an expert but I always follow peoples builds as well as doing my own van and if you buy a Syncro van, if its a rusty heap for £100 or a top end van for realistic money you have to commit yourself to a £15000 outlay.
Good luck with it though and if you dont want to commit to the £15000 then I would have a think about getting rid.
Its been said loads before but there is no such thing as a cheap Syncro
jed