Not sure what to buy  

I would recommend you to get Mini+, even printing flexi material can be problematic. But that's many times the case for any printer -especially if there's not enough experience. Yes MK3 is more versatile machine, but also costs a lot more and if you're not sure why exactly you need it, it's better to start with Mini -just my opinion.
I recommend you get semi-assembled version (get kit only if you think you're skilled with assembling and you have enough patience).

No right or wrong answer. I like both my Minis and my Mk3S. I find them to be pretty close in print quality. Surprisingly, at least for me the Mini turned out to be a bit more reliable than the Mk3S, after spending the $30 to upgrade the extruder with a Bondtech clone from Trianglelab, which I highly recommend if you go with the Mini.

My primary considerations would be:

* Size of the print bed. I print boardgame inserts and large functional parts frequently, so the larger size of the Mk3S is a big plus for me. Sure, you can try to cut models larger than the size of the Mini in bite sized pieces but it's extra work and prone to lead to issues. If you think you're going to be printing smaller stuff, the Mini's bed size is perfectly adequate.

* Direct drive vs Bowden drive. You mentioned flex materials. I found it MUCH easier to print flex with the Mk3S rather than the Mini. It's not impossible with the Mini but I've pretty much given up on it and simply use the Mk3S. As I said before, as far as print quality is concerned, I'd have a hard time arguing the Mk3S with its direct drive is any better, although you'll hear some folks argue the opposite. YMMV.

*  Price: You can buy 2 Minis for the price of one Mk3S. If it hadn't been for my need for the larger print surface, my most recent purchase would have been a Mini instead of a Mk3S.

Kit vs assembled: If you're reasonably mechanically skilled, the assembly is not impossible but it is a LOT of steps, and some precision is required. But you will learn a lot about how the printer works, which can help with troubleshooting. On the other hand, with an assembled unit, you'll be up and running pretty much right away, which I would argue is worth the extra money. Although of course there have been reports of issues with pre-assembled units, I'm quite confident that there are a lot more issues with units assembled by users. 

From your description of how you intend to use it, I'd buy the pre-assembled Mini and see how you like it. Less money to write off it you discover it's not for you. The state of the art in consumer 3D printers is certainly not that it's just plug and play and no maintenance/troubleshooting required. But with the Prusa and the support you get from Prusa and through these forums here you're going to be at the upper end of the spectrum. If you're into tinkering, go with the kit.  And if you think you're going to be printing a lot of large models and/or rubbery stuff, go with the Mk3S.

Thanks

Thanks for the detailed  responses , seems like the mini is the way to go minus being able to do the flexi stuff which for me was just a nice to have

Will start looking at that forum for the mods