I am looking at a set of rims, the bolt (5x114.3) and lug(12mmx1.5) nut specs are the same as for the RX300 but the offset is H rather than M. I am totally not knowing anything about rims, other than I want another set for my summer tires. Are these rims not compatible?

 

What size are the wheels and how "H" is the offset? L, M and H are inadequate offset measures - you need to know the exact number (usually in mm) or I'd pass on them - they will likely rub the suspension. Have you considered used RX330/350 rims - they will fit and look real nice.