Well- A twin needle industrial flatbed (I use a Consew 333RB at work) is great, but straight stitch only. You can make flat felled and rolled seams with attachments. I sew flags, and sometimes up to 10 or more layers at intersections when adding the headers. I swear if I stuck my finger under it I would sew through my bones. It uses 2 bobbins and 2 needles so each lockstitch seam is independent. Just remove 1 needle and bobbin for single needle sewing. Unless I were making sails I don't think an industrial zig zag would be useful.
An old Kenmore (model 82?) is fine for flatbed and cam sewing, zig zag, stretch stitch, etc.
Just add a serger and a free arm and I think that covers most of the bases. Some serious sewers like the older Bernina 830 or 930 (I think) for really nice straight, zig zag, and satin stitching. I personally like the features of needle down and center-left-right needle positioning.
A treadle or hand crank would be nice for nostalgic quiet sewing and power outages.
Keep an eye on Craigslist- I occasionally see industrial buttonhole machines for $150 or less here in NY.