The Arisakas are, essentually Mauser pattern rifles. There are lots of deviations from the Mauser, most notably the safety and action tangs, but it is the mauser system at its heart.
Many Arisakas were converted to other calibers in the US, because Jap ammo was non-existant, and commercial ammo in the Jap calibers was very scarce, and expensive. Today, you can get new brass for both calibers, so we don't have to form cases from .30-06 anymore.
Ballistically, the 7.7mm Jap is equal to the .303 British (same size bullet, same speeds), and good for anything the .303 is.
Prices on all the milsurps are going nowhere but up, and even poor condition rifles, unaltered, are bringing more than they used to. A complete "issue condition" type 99 (7.7mm) will have the monopod, action dust cover, and AA sights (except for the very late war variants), but are rarely found complete, and bring a premium price when they are.
Normal is a two or three piece stock (plus handguard), and the dust covers were often removed by Japanese troops (they tend to rattle).
An intact Crysanthamum "usually" indicated a captured rifle, not one formally surrendered, but there are apparently numbers of surrendered rifles that did not have the 'mum" defaced, so unless you have some history of the individual rifle ("dad/grandpa brought it back from Saipan", etc....) you can't really be certain.
Bores on the 6.5s have a reputation for varying widely from standard 6.5mm dimensions, so if you get one of these, check it carefully. Old loading manuals warn of gas blow by (always, ALWAYS wear shooting glasses) if the bores are oversize for the bullets used.
All else being equal (and it almost never is) bigger bullets tend to work better.