The Venture Prospector is indeed a good choice but if you're going for a polythene boat, try to check out the SP3 from Nova Craft before you make a final decision.
I've paddled a few Venture prospectors ( though never actually car topped or man handled one) and they are indeed okay, paddling pretty much as well as the SP3 from Nova Craft (I honestly think they're the best polythene canoe you can get in this country but I'm
no expert!), though theres something just that bit different about the SP3 that pips the Venture. The main reasons I'd go for an SP3 however, are the much better quality and standard of fittings and better re-sale value of the SP3.
The Venture may be lighter than the SP3 by a kilo or so but the points mentioned above would definitely sway the choice for me. Ideally try a few different boats before committing to buy but be warned-all poly boats are heavy and awkward to manhandle on your own
Two other bits of advice from me would be:
Buy 2nd hand if you can, you'll save up to maybe £200 on new, for a poly boat ( a lot more on royalex) and as long as its not too badly damaged in use ( by you), you shouldn't lose much ( if anything) on it if you decide to sell it after a year or two.
Get some professional coaching/instruction. It's invaluable and much better than either trial and error or learning from someone who already paddles but isn't a coach. I did but only after about three years of trying to learn from videos and books-I almost packed it in as a bad job, then got on a local weekend course with a level 5 coach-it was a revelation and I've never looked back since.
Good luck
Steve