First decide which platform you're going to run on. Since you're not looking for a full-on eCommerce solution, it sounds like all you need is a free cart.
I use VP-ASP, although I've tried Comersus as well. Both are ASP based stores, both are highly customizable and both allow you to update with additional features and functionality as your shop requires. In either case, you can use offline payment methods if you like. Unlike most solutions, the basic carts are free to use.
Check out the Sticky for a list of carts.
View the List
At any rate, be aware that you're going to have to do some serious template tinkering if you intend to change the layouts and doing so can be a can of worms.