Although I am a big fan of the small ring Mausers, particulary the Model 93, I would reccomend sticking with a large ring for a custom rifle. The exact action you pick depends on the caliber you plan to use.
If you want to use a magnum caliber or something the length of the 30-06, 270, 280, 25-06, etc., you need a standard Model 98 action.
If you re going to build something in .308 or maintain the standard 8x57 or 7x57 calibers, a Yugo would be a good choice, mainly because of price. The Turkish Mauser market has pretty much dried up. I have a Curio and Relic license and decided a couple of months ago to buy a Turkish Mauser. I couldn' t find one.
The Yugos are still plentiful, as are the Czechs.
The Yugo 48s and family are intermediate length actions. They are large ring, cocks on opening 98 style rifles, but the action length is the same as the Model 93s and 96s. Some sources say that makes them stronger than standard 98s. I don' t know.
I do know that up until fairly recently, they were a pain in the behind to customize. Nobody made aftermarket stocks for them. Boyds Gunstocks has solved that problem.
http://boydsgunstocks.com/default.htm They make a number of aftermarket stocks designed for the Yugos. The selection includes a nice looking Thumbhole varminter type that I have on my " need to buy" list as soon as I can break loose with a hundred bucks.
I have a Syrian Mauser which is a Yugo that was made for sale to Syria. The gun has the stock " sporterized" aka butchered, the barrel cut and crowned, a Timney trigger, and a Simmons 6-24X40 mil-dot scope. From a bench at 100 yards, it will put five rounds into one hole that measures less than half an inch across.
I shot that group a couple of years ago at the range in Laredo, Texas and still have the target to prove it. It is the only rifle I have ever owned capable of that kind of accuracy and is certainly a keeper.
I have thought about rebarreling it to .308, mainly because 8x57 ammo can be hard to find. But I have dies and have worked up some Nosler Ballastic Tip 220-grain handloads that are about as accurate as the Sierra 150 grain handloads I used to shoot that great group. So I guess I' ll just leave well enough alone. I keep about 100 rounds handy at all times.
Still, since so many people say the .308 is the mostly inherently accurate centerfire round available I may swap barrels somewhere down the line.
Anyway, I can heartily reccomend a Yugo for your custom job, unless you want to use a longer caliber. I have tried fitting 30-06 in the magazine of my Syrian and they won' t fit with 180-grain loads. I guess you could probably load up some 120 grainers or maybe 150-grain loads that would fit, but why limit yourself?