If you're never going to push it beyond 500hp, then any LS will do. If you want the most bang for the buck, then buy a 6.0/6.2. Both will be an extra $1k or so, but worth it in the end - if you have a sub 500hp goal.
The sub 500hp goal also makes most everything else an easy decision. You don't need a beefier transmission or rear - a 7.5 rear, TH350, stockish 200-4R or 700-R4, 4L60 will work. You can use a budget 340lph in tank pump in a stock fuel tank with the only the addition needed will be an inline post pump filter. Personally, I wouldn't mess with anything other than a Holley Terminator only because of the ease, inclusiveness and price. It will be cheaper to use a factory ECU and harness, but you then have to have someone modify it and tune it. When all said and done, the Holley will be just about the same price, if not cheaper. You need motor mounts and an oil pan for every LS swap.
Depending on your transmission plan, you'll need a bare minimum of $2500 and it's super easy to drop $5K. If your budget isn't in this range, then ........ hmmm.......ought to come up with a different plan.