Keep in mind that a Roadster will likely be more difficult to move on, and that the roof comes off on all these cars (see Pete's link and I've got a couple of posts in there). But I do think the interior of this Roadster would do it for me over the white coupes currently on sale for less money: https://www.pistonheads.com/buy/listing/15578199
Could also buy a hardtop, wing and front splitter to "convert" the roadster and get the best of both worlds, though Roadsters always look better than in reality than in photos.
Yes, you can buy a non-390+ car and retrofit all the required exhaust parts.
For drivetrain differences, see my post in the thread Pete linked (and quoted below).
Otherwise, pre-350 cars have the old gear linkage which isn't anywhere near as nice apparently (can't comment personally). I believe it's the facelift 350+ cars that have more nicely trimmed interiors (Alcantara etc.). Glass hatch on the older cars.
My post in that thread doesn't mention the exhaust bits required as I'm not actually sure. I believe everything after item 2 backwards is required but a specialist will need to clarify that.