Do you have this turntable ?
Just add KAB fluid damper and Isonoe footers, replace all wires, buy cardas litz or discovery litz (for tonearm) and Zu Audio Mission Phono RCA cable that you can buy on ebay every week for promotional price directly from Zu Audio (USA) under $150 max. Find yourself a good MM cartridge and listen to this deck first! This is minimum investment and maximum improvement you can do with SL1200mkII if you’re not gonna invest more than the actual price of the better turntable (SP-20 or SP-10mkII).
If you want to change tonearm you have limited space on the armboard, you need a new custom armboard, you have to measure pivot to spindle distance. You can mount any toneam with appropriate pivot to spindle distance if this arm can be fixed on the new custom armboard. That’s all you need to know, really. Internet is full of info about SL1210 mods, Jelco is one of the most popular solution, old or new SME is another one. I can add Luxman TA-1 tonearm (made by Micro Seiki) or Victor UA-7045 or Infinity Black Widow for extremely high compliance MM carts... But as i said any toneam can be mounted if the pivot to spindle distance is ok with the new armboard.
If you don’t have this turntable my strong advice is to buy SP-10mkII instead and invest to infinity to this (better) turntable.