2 different groups, Vintage racing and Historic racing.
Historic racing cars have documented history having been raced back in the day.
Historic race cars prices vary according to how rare, did it win a championship, current condition, was it restored to the correct and most desirable period in time.
They are expensive and considered an investment.
Historic cars will have results that may be found on sites like this.
Racing Sports Cars - Home Page
Vintage racing can be just as fun with recreations that look like they were raced back in the day.
Whatever vintage car you race I'd try to find one that can be made extra safe.
Spectate a few vintage races and talk to the drivers in the classes that you find interesting.
Find out what modifications are permitted, or good cars that are for sale.
Some other resources.
Sports Car Digest - The Sports, Racing and Vintage Car Journal
THE VINTAGE AND HISTORIC RACING NEWS MAGAZINE
Vintage Motorsport - The Journal of Motor Racing History
Find out what vintage race clubs hold events close to your location.
I like to race with these 2 clubs.
SVRA | Sportscar Vintage Racing Association
Historic Motor Sports Association
Vintage or Historic racing appeals to me as I really don't care to do track days with new cars.
In car video at our local historic/vintage race.
https://youtu.be/xksW6hYJFS4