I have the same ambition after racing in a few other 24hr races. Timing and commitments keep playing against me
. Maybe next year!
My advise would be don't enter your own car. 24hr races are like a season of racing in one weekend, no mater how cheap you want to make it, it will cost thousands and much more than you think.
Assuming you have'nt done the Ring before or maybe if you have a few times I found a good build up option is do the 75 Experience at the Ring, learn it inside out, Ron (who runs the experience) is also one of the assors for the 24hr Ring "newbies". Then buy yourself a seat in an existing entry.
To find a seat watch the race cars for sale web sites or contact the teams directly that raced last year and see what they have to offer. You can find them with a bit of web research.
Cars like the Honda Civic Type R's, Alfa 147 and Seat Leon Diesels, or if you can stretch to one BMW E36 M3's are the best options for investment to performance. Although cheaper or much more exotic options do exist of course. Either way seats can be bought fairly easily for upwards of 3500EUR.
Maybe see you out there one day!