To be honest I wouldn't go with converting a mini mill for this work. End of the day they are toys and over priced toys at that. You would basicly end up scraping half the machine just to get it to perform any thing like acceptable. Even then you will still be left with sloppy dove tail ways needing constant adjustment to keep accurate.
With your knowing exactly the purpose of the machine and your small work requirements then you could build a far stronger and more accurate machine custom to your needs that use's quality profiled linear rails, ballscrews and all the trimmings for less money than buying and converting minimill.