The Supermatics are tough and strong machines up to most jobs, don't underestimate them. I keep mine for the double layer cams, freearm and capability for larger size needles. I have an eye out for a Necchi BU too, but I have yet to find one near me. The BUs are well worth the money and effort of fixup, they all metal, very sturdy construction. If it meets your requirements I don't see any reason to hesitate.
My memory of a black BU is a super smooth machine, easy to use, and stitching effortlessly through layers of cotton canvas, flat felled seams, etc. I would guess it's all about cleaning and oiling to start with. For freemotion there there are usually both vitage and new jumping feet availabe. There might be a bit of trial and error for the feet with spring action, but you will find something that works for you.
You should get far with basic cleaning and oiling, take the bobbin case, race and hook out, the off the bobbin cover and throat plate to clean. Scrape corners, grooves and every tooth in the feed dogs clean with a tooth pick. Take off the face plate, clean, and detect all oil points and give them a few drops of oil. Where the needle and presser bar goes up and down needs oil too. The same for the cover in the back, most holes on top of the machines are oil points, and there are lots under the base. Every joint, hinged part, where metal move against metal needs oil. A BU is luckily relatively simple for a DIY fixup, and the most likely replacement parts are motor belt and bobbin tires. With a bit of search you should be able to identify the correct replacement tires for the winder and the wonder wheel.
Last edited by Mickey2; 01-20-2019 at 01:44 PM.