Good beginer table, that I can upgrade?

A great TT is the Denon 47F. Upgradable? No. In the used market these go for about $250 but only one that can be picked up is worth buying. The only upgrade that will be necessary is a good record clamp, a SOTA. When you want to move up better to keep the Denon and buy one of the VPI machines.

Even if you are talking about only the table itself, no arm or cart, I can't think of any choices in your price range. Possibly an older model from Oracle, but again, in that price range? If you want an arm and cartridge as well for that $300, forget it, not going to happen, I don't think. There aren't really a lot of table manufacturers who design tables to be upgraded in the first place, and the ones that are, start way above your budget normally. The best example I can think of would be the VPI Scout, but their starting price is multiples of your budget even used. The good thing about buying a table in your price range, even if you can't upgrade it is that if you do decide to upgrade in the future you should be able to recoup the bulk of your initial investment when selling the table. This would be much different than say spending $1500 to $2000 for a table that can't be upgraded, and then wanting to move up later.

You could find a technics 1200 and do kab mods over time or an ar es1 which you can then do the ms mods to. Either deck can provide extremely high performance when dialed in.

...is there a cheap but GOOD turntable

No

You can do it. The trick is to see that your final turntable, years away, won't have any of the same parts in it. You'll upgrade in the best bang-for-the-buck order and at one point the cartridge will go, the arm will go and the turntable will go, replaced with something better.

I like a Goldring GL-1 or GL-2 or a Rega for tweaking. At your budget level you'll have to find it used. There are lots of little bits and pieces you can get on eBay for making the motor quieter, damping the bearing shaft, and so on. You can upgrade the stock cartridge. You can rewire the tone arm, then shift arm and cartridge to a better table later.

You can't easily sell the original tone arm, even rewired, so when you replace it with a new and better arm, you shift the rewired arm back to the original table and then sell the old table plus arm and cart as a unit.

Voilà, an upgrade track for a turntable that's "too cheap to tweak".