If you go for a commercially made boiler then £5-6000, maybe £1000 less if you make your own boiler.

I've built a 1" Minnie and 2" Fowler.

I came to a bit less for the castings etc that Blackgates list but the price list is 2018 and comes in around £2500.

Last time I heard the price of a 2" boiler it was just over £2000 and don't expect the price to drop.

You then have to buy the rest of the materials, sheet, bar, fixings, some boiler fittings say £1000 – 1500

Don't know what machines and tooling you have but expect to have to buy some tooling for specific jobs and certainly cutters, taps & dies etc say £500.

If you are a member of or join a club they will do the testing as part of your membership but are obviously not keen on those that just join for that reason only, you also get insurance via the club. The other options are to have the boiler tested by an independent boiler inspector which would cost a couple of hundred pounds plus separate insurance. Certificate will need renewing each year.

Yes it is possible with the Minnie more than most as it is not a Scale engine, could just buy the flywheel and maybe the cylinder.