I think it's a good price for what your getting, the RO will probably only just be enough for tops only, when an RO is sold as 100gpd then that is the most it will ever do under ideal circumstances, so if you are thinking of using more water later then get a better RO.
Backpacks are normally ideal as an addition to a van mount, there are some on here that work only with a back pack and nothing else and get on fine. If it was me though and I had the choice of back pack or trolly then I'd probably go for the trolly. A backpack that size will normally squeeze out a house and a bit unless your a real scrooge with the water.
Simon.