Wow, big list! With regard to engines, this is about the smallest transplant i've seen and I did it!
Its a 2ltr 8v Vauxhall Carlton engine with Omega gearbox and Dolly sprint axle. It gives about 130 bhp in its current trim which is plenty for road work!
MX5 may fit, I know someone who has one in a Vitesse and thats pretty sweet.
The problem with a K series is finding a suitable gearbox (Morgan did but I don't know what it is!)
Dolly shells are all basically the same, there are only detail differences, the only holdout is the Toledo which has all different panelwork aft of the rear window.
There's no really simple brake upgrade as Sprint Discs and calipers are the same as your 1300 ones, however the Sprint has different pads and a much bigger servo (the Sprint rear brakes are much bigger too, actually TOO big!) The TJ conversion is on my shopping list but I don't know how much it costs!
TR7 rims will fit the hubs but I don't know if there are clearance or geometry issues. I've used MGF 15" alloys, pretty successfully as you can see, but they're not everyones taste.
If you don't fancy something as big as a 2ltr the Vauxhall engine also comes in 1300, 1400, 1600 and 1800cc variants which will all fit (with some mods) to the Omega box.
Since you're skint, I'll add this, I did mine all myself and already had the engine but I had the whole project on the road for £600 which includes the £200 I paid for the car!
Hope this helps1
Steve
Ps VERY few Sprints had the very expensive LSD option and they are highly prized and very expensive today!