Can I fit 20 inch rims on a 1993 Lexus SC 300? What size and tire would you recommend?

your car uses a 25.3" tall tire and the shortest tire for a 20" wheel is 26.8" tall ... so, I would say no a 20" wheel will not properly fit your Lexus

Just to make sure will a 18 x 8.5 rims with bolt patter 5 x 114.3 fit a 1993 SC300.

for an 18x8.5 wheel you would need about a 40mm offset ..... but, you could very that about 3mm either way

20" will fit on your sc300 if your car isnt lowered. use a 245 45 20 on the front and up to a 275 30 20 on the rear you can lower the front using an eibach coil spring ( pro kit only ) 1" the rear end doesnt need to be lowered.