Is there any car you haven't considered/bought?
Be a tad careful about the MG, it may have been remapped - but the ECU may not work in the Freelander and if it does your security/immobilisation won't work fully so your insurance, as well as going up for a modded car, will go up again.
Plus it may be a mapped VVC engine - but it won't exactly set the world on fire in a Freelander. I'm pretty sure you won't get great satisfaction out of it. The only way you can get satisfaction out of tuning a K Series is to put a Turbo on it. It'll be an instant change - but once again, that's about all you can do to it. As an ongoing project, well you're back to just fixing head gaskets!
The V6 is a lovely engine - drove 1 for the first time a couple of weeks ago and its a very pleasant experience. Once again, not sure what you can do to it other than perpetually trying to stop it overheating and pouring copious amounts of fuel into the tank - not exactly creative cash usage for your project.
The more I think about Freelander - the more the L Series is the ideal setup for almost all scenarios! For your requirements as a project its super
When you get it, you can spend some time giving the engine some TLC - but going forward as a project, your work on it won't be boring fixing of faults. Unlike the other engines, there are all sorts of performance changes you can make to it - there really are loads of options for you to have a look at and play with. Its never going to be a Veron, but you can via a cocktail of changes almost double the engine power to to around 200HP. In Freelander form, the L Series is quite a bit slower off the mark than the K Series, but in Rover car form, they are very close, and a bit of tweaking leaves the K Series for dust (well actually in a cloud of smoke). So there's all those things to play with - and the engine bay on the L Series has good access to tinker around in - the K is OK, but the KV6 is comparatively a nightmare.
As has been said as well, the L Series are the early cars, so basic and rather utilitarian. Great scope to add what ever you want
This fella's had great fun turning a couple of L Series into his project cars....
https://www.muddymods.com/pages/about-us
Depending on what you want to achieve as well, you add the fuel into the project as well. The L Series injection system will allow you to bung almost anything that'll burn in the tank. So if you want to start processing cheap used chip shop fat into fuel for your car, the L Series is the way to go - try putting anything other than premium diesel into a TD4 and it will spit its dummy.
Just some thoughts!