If you plan to do any oval racing at all, a wheel is absolutely essential - if you don't like oval racing, don't write it off.. some of my best racing done on iRacing was on the ovals.
For road, a controller may get you buy on the MX5 but it is a severe handicap.
Personally, you should probably run a few test sessions until you can turn a few laps without incident. What I was doing was going into practice sessions with other cars and getting to learn how to race in traffic. iRacing is unlike any other racing sim/game. Can't argy bargy your way through the pack, you need to remain clean as possible.
On Oky when I started out, I used to aim for consistant 1.05's. On Lime Rock, I used to aim for 1.03's. Best on Oky is a low 1.02 and Limerock is a high 58 - the aliens can shave at least another second off those.
These are two pretty tidy laps so you can observe braking points and apex for Oky and LRP.
http://www.youtube.com/watch?v=l52hX1kG9i4
http://www.youtube.com/watch?v=Ek0BMQH1Atk