Not really sure what to do at this point. Invest & rebuild her or sell her & start again?
You need to work out what the ongoing 'real world' costs are for both options if you wish to keep a Mini as a daily driver for both winter and summer.
Go look at some actual Mini's up for sale in your price range and see what you could get for your money and what you would get back for yours in its current state.
Then get some accurate quotes for the work needed on your existing Mini Matilda, especially if it is not to be a diy project. Also how long will it take to do if you will be without a car for transport. In my experience whether diy project or a specialist restorer it always takes longer than planned and costs more!
Either way you should also have a contingency fund for those unknowns...........you mentioned gearbox but this could fail on yours or any similar replacement Mini.
Of course in my past days of running Mini's as daily driver's, we fixed them up and modified them, drove them for several years to about the stage yours is now, sold on cheaply or in worst cases broke it for spares.......then bought the newest, affordable, good condition replacement Mini we could find, transferred all our mods/goodies over and got another 5 years or so cheap motoring. However in those days Mini's of all ages were plentiful and cheap.........