Ribbon is super easy to source actually. Most models use a standard width for the ribbon itself and someone invented a universal spool that will work in about 95% of them. Unless its really old (pre 1920s ish) youre going to be able to find someone who sells ribbons that fit the other 5%. You can even find ribbons with fun color combonations. I was just typing on a green and brown one to go with my hedgehog notecards. Worst comes to worst you can transplant ribbons onto other spools, its pretty easy, but a little messy, I reccomend wearing disposable gloves.

Regarding the machines themselves...

Olivetti Lettera 32's have an excellent reputation for holding up to the test of time and suiting modern users needs. Same goes for Lettera 22s. I'm totally not biased at all.

Larger machines will be more comfortable to type on and may have some extra nifty features, but most people seem to prefer compact models for the convenience. Only the super deluxe models are going to have a key for 1s and !s. On all others the ! is going to be a combonation of an apostrophe and a period, the letter l doubles as a 1 as well.

Before buying you should type a little on it to see if the keys and functions work properly. Also check ebay for price references but not etsy, for some reason the prices for machines are severely inflated on etsy. Most vintage typewriters are fairly well made and will still function well unless they got the shit beat out of them or were exposed to the elements for an extended period. These things also require very little lubrication, if any.

Rubber parts are definitely going to be significantly degraded, however.

visual references spoilered for reasons.

The biggest victims of rubber degredation are the platen and the rollers, this is what feeds the paper through the machine. Unless replaced/restored a vintage platen is going to be rock hard by now. Thats not so big of a deal. The quick fix there is to have one or two extra sheets of paper "backing" the one youre writing on. This will "soften" the typing surface, helping with print quality and noise concerns. The rollers however, could be fucked up. If kept in the locked position for decades they can warp and develop divots that will harm their ability to feed paper properly. Everything will work... kinda? and youll have some issues with things like feeding or spacing, but its nothing that cant be worked around.

There still exist typewriter repair shops if youre willing to shell out the cash for service and shipping, or you can try your hand at repairs (this is fun, really). There are guys on the internet (ebay and etsy are good bets) that 3D print replacements for now-desintegrated rubber parts, especially the feet. Guides on home-made fixes for hard platens and warped rollers are out there too and they're really not that bad as long as you dont lose parts and screws. Youtube is your friend, and if you have an Olivetti you will have an abundance of resources.

I also reccomend finding a box or bag or case or something to cover/store it in when not in use. These things dont like dust. Vintage cases can cost more than the machines themselves, a reasonably close fitting cardboard box will do. Finding some sort of pad to put under the machine while in use is a pretty good idea too. This makes operating it a bit quieter and may prevent it from shifting around on a table. There are some very nice cork/felt ones out there but theyre just nice to have, not critical.

Also liquid white-out that comes in the bottles with the brushes? Like a dollar each and indespensable for typewriting.

Pardon the endless word vomit. I like typewriters.