If I was going turbo on a M20 I would look at starting from scratch and building another engine up with all new seals & gaskets etc.
For sensible power, i.e around the 300 mark, I reckon you'd get away with stock internals all day long, just an uprated headgasket and some ARP head bolts.
Exhaust manifold is the pain as far as "off the shelf" is concerned as most of the ebay type stuff you see is generally made for LHD cars. ALso, you are into stand alone engine management like Megasquirt etc.
Without kidding yourself, I'd say you'd be into £3-£5K to do a decent job of turboing the car by the time all the little bits & pieces are added and you've also checked that suspension & brakes etc are in good order. I reckon I'd want more than just an uprated set of discs & pads on the front too.
Essentially, the figure you probably have in your mind can more than likely be doubled and then some
