Welcome Bolts. Enjoy the hunt for your car.
Regarding which is better; it's swings and merry-go-rounds in my books. Yes the L24 revs faster so sounds good and can be a little more responsive. The very early 240s are also lighter.
The 260s are a bit stronger in the chassis, so are possibly better for engine swaps. There are slight differences in the suspension that can better support the increased weight.
As far as engines go; the L24 spins faster - which is fun.
The L26 is an engine I wouldn't use as it doesn't spin as fast as the the L24 and doesn't have the HP or torque of the L28.
The L28 is simply more powerful in HP and Torque than both the others. It can be made to spin pretty quickly by lightening some heavy parts - like flywheel. Some of the guys with big budgets are getting some impressive numbers out of an L28 NA - there is always the option of turbo.
Again it the $1M question of what you want to do with the car???
I would suggest getting your head around ALL the Targa rules before you do anything. If that is what you are interested in doing with it, the rules will guide your decisions.
I, like you, prefer the 2 seater's shape, but there are some quick 2+2s out there. I'm sure I've even seen a pretty competitive 2+2 in the Targa. The cheaper car purchase can then offset the cost of some of the mods you want to do - if money is ever an issue for you