I just wandered in on this one. I have a pair of JH's, one stock, one, well, not so stock. One nice thing about the car, is unlike a lot of it's contemporaries it's surprisingly large. It's actually bigger than a Miata.
#1. A well sorted JH, even in stock configuration is a blast to drive. The *only* real downside is the freeway RPMs. Both the 4-speed and the 5-speed have the same final drive ratio and that means 3-4k RPM at full freeway speed. And surface streets I can happily slide my car sideways around corners (which tends to freak out other drivers on occasion).
#2. If you're going to modify it, I highly recommend staying with the 9-series engine. In stock configuration the weight balance is beautiful. It's not a heavy engine so it's hard to find decent swaps that are worth the effort. the other item to keep in mind is that the thing is the size and weight of a rollerskate. Since the real measure of a car is horsepower to weight, you don't need 1,000 HP. The current build I'm finishing includes an upgrate to a 912 block out of a Lotus, 2.2 crank, pistons, half-moon pulleys, flowed head, and dellorto 45's. I'm also swapping out the stock transmission for a toyota supra one. You can get a bellhousing out of one of the Lotus Eclat versions that will mate up to said transmission (the bellhousing version depends on the generation of the 2.2L crank). Then it's a matter of having a shaft made. Conservatively should make 200-250hp, which is a lot for that car. Also, I have seen JH's with rootes style superchargers on them. That's something on my list of "some day" projects as it requires fabricating the intake from the supercharger to the head.
The only downside to option #2 is that the build isn't cheap. But consider that Lotus, at the end of life of these engines, was producing somewhere in the range of 300HP from a supercharged version of this engine in the Lotus Esprit S4.