You can add Penske as a chassis, as for most years, they had their own chassis. I did a few searches, and it seems like 1984 was the last year of having a many one offs (Eagle, Longhorn, Wildcat, DSR-1.....). So like
@jymp said, maybe anything after that. The other thing to maybe consider is that some chassis was carried over by privateer teams the following years. For example a 1987 Lola was racing in 1988. I'll have to look better at the numbers but this I don't think was as common as it was in the early 80s to the late 80s.
So doing a bit of thinking...
1984 - Chief started this for rF1. The models could use a small rF2 refresh (it's been a long time since I've looked at them) Con: More one off models to complete whole season
1985 - Lolas and Marches and Penske. Existing 1984 models mentioned above can complete the grid.
1986 - Lolas Marches and Penske. Could add a special, never raced car
http://www.motorsportretro.com/2014/05/ferrari-indy/
1987 - The 1988 HSO contains a few 1987 cars. Needs approval and needs to be converted to rF2
1988 - Season fully complete by HSO. Needs approval and needs to be converted to rF2
1989 - The 1988 HSO contains 1988 chassis that raced in 1989. Needs approval and needs to be converted to rF2
But at this point, anything to race on both ovals and road courses would be great!