really the cost of the COnverted mill is high if you plan on making your money back with this only from your company. i would look around for a good used mill professional series cnc mill. i have both a cnc IH mill and a manual IH mill. both were great learning experiences when i started my business. the biggest drawback to these machines even the tormach is the lack to tool changer which i find to be invaluable at this time. but to answer your questions...
1.) Is there any difference to the IH Manual Mill vs the IH CNC Mill after conversion? (Besides the quality of the worksmanship of the conversion)
no there is no difference between the two except the time that it took you to convert it and the quality that you put in it as opposed to what tommy from IH will supply you.
2.) How difficult/time consuming is the conversion. What are the extra $100 in electronics the conversion kit is missing? I'm comfortable using a mill, I'm admitedly nervous about risking $7K because I don't understand some instructions.
the conversion is very time consuming. i would plan on about 2 months. granted the mechanical conversion itself is not that difficult, but attention to detail is required. i had made some custom spacers and custom shims etc for my mill to get things just to that next level and there were still some issues. (LOCTITE EVERY BOLT) that was one mistake i made never to repeat again, since some bolts you can't get to once other parts are assembled. also you will then have to tune your own motors and build your own electrical cabinet and do your own wiring. i had to redo my wiring twice. once was to get it running the second time was after i figured out what i actually needed. i redid it and its better. i still plan on adding a control box with switches that i have to build.
3.) Even with the VFD the max spindle speed is rated 3600rpm, has anyone found this limiting? (Most of my work will be on AL6061 and C1018, maybe some 316, and wood)
if have my vfd pushing 4500 RPM with the high gear. although this isn't what is sold it is what the mill can do. the low spindle rpm isn't that limiting but faster would be nicer.
4.) How helpful is autofeed? (Useless if I plan to convert to CNC later?) useless if you convert to cnc.
Again really what you pay for in the cnc package is the electrical control cabinet. if i had to do it over again i would buy the ready to go unit as opposed to building my own. but like you i though i would save a few bucks. what i ended up saving in money i wasted way more in time trying to trouble shoot errors and making adjustments that could have been more productively spent making parts. if your going to use this to try and make some money then do yourself a favor and get a ready to go mill. if your doing it as a hobby then get the kit.