I didn't think the small Model Masters like that cost quite that much unless something changed very recently. Are you sure that $24k isn't a package with Artcam included? I'm pretty sure that mill was a fair bit less than that when I bought mine, but that was a few years ago and they have added models to the lineup since then. $6k for a non-runner still seems steep.
The total weight is likely around or under 60kg, they are a pain to move alone but fully possible.
If it isn't working, you may not have to remove everything and start over. Mine, and all I have seen, simply used rebranded Flashcut systems and control software, don't know if they all still do but I'd say its still very likely. If so, repair or replacement of missing control components is very possible.
I think you might be better off buying a different base machine if you are using a Sherline low speed spindle and the kind of cutting that entails though. That machine was designed to use a very light high speed NSK spindle, and to move it quickly. They are beautifully made machines, but they weren't designed for heavy, slow milling loads and so uses very small fast single-stack motors. It isn't just going to be as easy as simply hanging that heavy of a head onto the Z without other modifications as well. I do not remember what the pitch on the ballscrews is offhand, or if it is the same between all axes.
If you are going to go whole hog modding it though, that machine modified for use with a good set of servos and a properly set up ER16 spindle would be a dream to use. They are very nice machines and very well built, just not exactly built for the kinds of job you want them to do!