to be honest, if you have your site up and running and have spent time you don't want be wasted, have a look at the add on's for the 1 page checkout, as could well a way to solve your problems.
You just have to weigh the cost of buying it against loss of customers (or go with something different.)
Personally, I expect to have to pay something towards a shop, but there are other issues where i am concerned with prestashop (which i accept is down to me not being a programmer) which has made me look elsewhere - but that has cost me (acceptable to me) about £180 for a basic setup, is more work than prestashop, but more flexible, but you do need to be aware of html, php mysql etc, (and the more aware, the better :-)