You might want to hold on to the flatbed. I bought a Singer 201 in a small table a few years a go and it is still my favorite. It runs smoothly, it's very capable with every type of fabric, flatbed in a table makes it a joy to work on. Even though it's a standard domestic model, it's comes off as large with lots of space to maneuver bulky fabric pieces. The space on some models can feel a bit cramped, others come with a luxuriously long freearm.
I agree on the freearm, I keep one with option for various stitch patters; though I mostly use the straight stitch, zigza, blind stitch, and the odd stretchy stitch. I think Bernina still have as narrow freearm on the new models and the older, it has been an advatage a couple of times. If you make clothes or mend clothes, attatching a piece to a round opening, cuffs, can be a bother on a flatbed however nice it is in every other respect. I like the buttonholer attachments for my 201, they work very well.
I have to say, there is no such thing as a one step buttonhole; it always involves quite a bit of test sewing and adjustment for thread fabric and size. I don't mind the four step too much, but when it's been a while I have to look it up to get the hang of it again too. Learning how to do it manually isn't bad either. That said, the one step buttonholes are nice, several shapes at a touch of a touch screen (in real life a lot of touches and a bit of test sewing, trying to get it to stitch over bumps and layers at the edge of a duffle jacket...).
I went for fixing up a couple of vintage machines, it turned out to be half the fun too. A Bernina 330 sounds perfect with a freearm and nice buttonhole selection. For buttonholes I have discovered very few handle top stitch thread, it can be nice on some fabrics. It shouldn't be any problem in general, at least for the models you mention.
A side for making clothes I sew a lot of odd things like cardboard, experimental plastic innerlining, leather, bike seat covers, and I would not dare to put a new Bernina 330 through struggles like that. That's when an old all metal cast iron machine is nice to have, and I have to admit I push my freearms to the limit too, but they seem to hault before they are overloaded.