I don't have anything against you wanting to turbo your M42...
I just find it comical of some of the points you're trying to make that's all. $$ for $$ the S52 is a better swap. If you do want to turbo your M42 it shouldn't be for fuel economy reasons. Ask anyone that drives a boosted car, it's very addictive.
In the end you should boost your M42 if that is what YOU want. But think long and hard of how/why you want to do it. Set a realistic power goal and your budget will then fall into place.
Consider all the hardware parts, along with things like clutch, fueling, management, and tuning costs as well. Keep in mind things can/will probably go wrong the first time around especially if you're going to try and save some money and go with used parts.
This isn't my first time around the block when it comes to boosted cars, so I'm trying to give as useful info as possible. For your power goals and keeping in mind a budget and reliability the S52 route will always be the better choice that is undeniable trust me.