What do you play now, out of interest?
The beauty of coming to sim racing now is I have been able to pick up quite a few excellent sims that are now a few years old, so cost hardly any money (if you get them on sale).
My favourite is Automobilista 1 (AMS1), which I picked up for £5 on Steam sale a few months ago. I love its physics and car handling and it has very good AI that are very convincing to race against. Plenty of excellent free mods available.
As goalgerd mentioned above, Automobilista 2 exists - it cost £25 for early access release but I think it goes to full release very soon, so price will go up to about £40. It uses the same SMS engine that PCars2 does, so looks great and has the same excellent full dynamic weather system, etc, but seems to carry through some of the iffy physics and AI from PCars2. Reiza Studios have been working on improving that, but for me, they still have some way to go. It has the potential to be the best but I currently much prefer AMS1.
Assetto Corsa - picked this up as part of a bundle of games, so effectively cost about £1.50. Car handling is great, AI is adequate, loads of brilliant free mods.
RaceRoom (R3E) - costs about £55 for the full package but very good sim. You can try it for free - you get a few cars and tracks. You then have to pay for additonal content.
rFactor2 - I only have the demo and am waiting for it to go on Steam sale. Love the handling. Physics and AI are reputed to be amongst best in business. Apparently can take a bit of faffing around to get the best out of it but I'd rather have that than a no-faff game that only ever plays like crap.
I haven't tried iRacing but it's supposed to be very good - however, it's subscription only so works out very expensive in the long term.
Also haven't tried Assetto Corsa Competizione (ACC) - also supposed to be good but I already have more than enough to keep me entertained.