You need to go to the ABRA website and click on the rules section.
Actually, Rugers are not required; the LVT simply is the most generally accurate model available under the factory rules. I understand that until it came out, the Remington 597 was a serious contender. The rules actually state that it must be a factory autoloader, have a barrel no larger than 0.75" at the muzzle, be fed from a magazine (no manual loading), and the barrel be factory marked as to specific model. I haven't actually seen the factory Kidds and VQs but I think there are two places they don't qualify for factory. I suspect they all have barrels larger than 0.75 at the muzzle, and that the barrels aren't specifically marked as to model. If not, there is no way to tell if the barrel has been replaced or not.
The 0.75 rule is what disqualifies the Rem 597 HB. Under the rules a factory gun can be rechambered and recrowned. The stock can be rebedded and the trigger modified in any way as long as the original housing is used. The LVT, being in essence a Ruger target model with a lighter barrel contour and regular stock, is the best basis for improvement. It already has a target chamber, although generally too long. Shortening the barrel tenon slightly and recrowning is simple and effective. Installing a Kidd trigger kit (or a good trigger job) and bedding of your choice yields a competitive rifle. They are not the only option, just the easiest and most available. I have seen one T/C that is shot in both factory and unlimited and scores well in both. Another one I have seen is a 50 year old Beretta that is deadly accurate. That being the case, I would be surprised if an early Italian Weatherby wouldn't be competitive. The Beretta mentioned above while practicing actually shot a score in the 190's when being single loaded by hand. I think improved magazines and feeding are the next great step for all these guns.
Good factory guns score around 180 on the ABRA target, as compared to the low 190's for the unlimiteds. ( I am talking aggs here, not one target bragging. ) To be competitive, you will need a factory gun that can agg in the high 170's over the long haul, or an unlimited that can hit 190 as a minimum.
The unlimited guns can be anything, period, as long as they autoload. As a rule, these are all customs at the top level, because even guns like the Kidds and VQs have compromises built into the chambers etc. They have evolved to a very high level of performance, given the platform. Stocks are one big issue, since these guns really perform best with a true benchrest straight line stock, and these are not available with a Kidd or VQ, so there is a big financial penalty.
As to ammo, the rules state only that it be available over the counter. There is no requirement as to where it is purchased. CenterX seems to be the single most common choice in unlimited. Lapua in general dominates, I suspect because of the nose shape of a lot of Eley not working well with autoloaders. Due to the generally more generous chambers, factory class has a wide array of ammo used. You need to have ammo that chambers reliably and partially engraves the driving bands.
Hope this helps.