Bit of thread drift here!! I don't think our original poster contemplates an Extra 500
Strikes me that £70k + £600 a month gives a huge number of possibilities. The thread started with talk of an "IR machine". I am curious to know what he contemplates exactly as to use an IR machine fully you need an IR! Does the type of flying you are contemplating really involve significant amounts of IFR flying?
What seems generally true is that the majority of pilots fly far fewer hours that they plan to when they buy their dream machine. I have seen some very nice aircaft standing idle as a planned 100 hours per year becomes 10 to 20. That translates into a very high cost of maintenance, particularly on a certified aircraft and £50 per hour at 100 hours becomes £100 + as the annual is still coming out between £3k and £5k per year.
A small well run group is an excellent way of discovering if your lifestyle allows you to fly as much as you plan. The difficulty is in finding that group and then of course you are constrained by what is out there within a reasonable range of where you live.
Aviation is a hobby, not an exercise in home economics and only you can say if you are comfortable with the expenditure, but do keep something in reserve, especially if you are buying outright. I would not worry too much about the future of 100LL but if this is a factor a lower powered engine will be a better bet for a future of 100UL than a high compression 250/300 HP variant.