If your only looking for 150 bhp (would have thought your G40 would be slightly more than that) then a basic well set up 1.8 16v is close to that, and stick a 2.0 bottom end on that and its around the 170 mark. All done very cheaply too, those things sing right round the rev range and will take a hammering all day long (well mine did when i had it in my mk2 11 years ago)
Its a bolt in convestion on a 1.5 and above shell, tried and tested route. Plus spares and repairs are much cheaper. Large choice of boxes too
You will be surprised how fuel effiecent those engines can be too.
Then there is also the G60 as you mentioned, people have been getting 240+ bhp for years and costs are coming down of these, fairly plug and play
You also have the 1.8T which can be had for next to nothing now days, running golfs are down to the £1k mark now, sell off what you don't need.... Standard tune with a chip and they are 180bhp and you can go up to 280bhp for under £1k later if you feel the need. Don't buy a Qpeng loom, speak to rubberjohnny of ED38, he makes looms for these and I think they are around the £3-500 mark. Yes may cost a little more money than the G40 route, but very easy to get more power out of them when you get bored, which you can't do out of the G40 (what for 200 bhp will cost you £1-2k, whereas on the 1.8t that will get you a reliable 270-280 bhp and change!!!)
THen if you want more of a challenge a VR6 and Turbo will fit in, though you might want 4 wheel drive then!!
Don't get me wrong, I think the G40 is a great engine, and run one as a daily and building a mk2 coupe G40 but for the Golf there are better engines.
Good well know and strong G40 engines are always sell, its just the poor advertised unknown engines and cars which are taking time to sell. If your prepaired to break it up it will sell quicker too. I might be interested in a few parts ;-)