If it was my money (and it somewhat is as I'm facing similar challenges).i would go right for the heat treat oven. Reason being, a pretty knife is just that... Pretty. Means nothing if it's all looks and no cut. While milling machines help in knifemaking, it is not a necessity. Proper heat treat absolutely is a necessity and consistency in heat treat is far more important than looks in my opinion.
Extra elbow grease can make a milling machine obsolete, many do without it. I've been heat treating using a forge and while it works, it is by no means exacting. I am looking at a heat treat oven as my next purchase because I feel the HT is the heart of the knife. Plus, for the cost of building a homemade oven, you may be able to get both with just a little more saving and some fabricating work. The tutorials for homemade ovens seem to be simple enough with a little bit of electrical knowledge, so you may want to look into that route instead of potentially 5x the price for a commercial unit.
Food for thought...