If the second hand engine has good compression I would leave it in the state you purchase it,opening it up is wasted money,If by some reason you damage it while tuning then rebuild.=) Like most transmissions,if nobody changes the oil/filter they fail,fix it only if it fails and dont worry about it until then.
I know very little about the coswoth engine,but im pretty sure they ran solid lifters and different valve spring type,you cant directly compare this to the M111 head with hydrolic lifters,the wiegh more for a start.
Cam design is really speciallised,engine builders normally dont have the software or equipment to sort these issues,locally we have companies like this: http://http://www.kelford.co.nz/ If you can,try get away with using stock cams but remove some of the overlap buy adjusting timing,it may depend on how much piston-valve clearance you have,if you cant get away with it this way you may have to adjust duration etc which is custom cam design.

The best thing you could do would be to speak with someone who has turbocharged a non-turbo engine many times(not an ECU tuner),have the M111 engine degree'd and descuss what possible changes you might want to make before you start the project.

5k is not alot for a completely custom turbo project,but im sure there is a long line of people who will tell you it can be done for next to nothing!.=)