The Enfield has the advantage of being the fastest bolt to operate of the three you mention (check my other posts for the fastest bolt manipulation method) the Mauser second fastest and the Mosin Nagant third.
The safety on the Enfield and Mauser are about the same in speed to disengage the Mosin Safety can be a pain to operate unless you retrofit it with one of the cocking pieces that have been modified on the back end with a welded D ring.
Ammunition cost favors the 7.62x54 rimmed ammunition for now with surplus 7.62 NATO going up in price and becoming harder to find.
8mm Mauser can be found but is not always relilable much is 40-50 year old surplus its not always cheap you can buy current production ammunition which can add up to money real quick and most 8mm Mauser factory ammo is not loaded to full specs just check some ballistics charts.
You can take off the wood stock on the Ishapore MKIII 2A and replace it with a couple different fiberglass stocks also a Jungle carbine variant turns up from time to time such as those being sold by AIM surplus IIRC right now.
The 8mm Mauser and Ishapore SMLE MK III 2A are fast to reload one holds 5 plus one in the chamber the other holds 12 plus one in the chamber.
The Enfield has the advantage of the detachable magazine spares are available from Gun Parts Corporation/Numrich Arms that hold 10 rounds they are almost identical to the factory magazine, M-14 stripper clips seem to work as well.
The Ishapore if it has a good bore and headspace can surprise you I have a couple that go under one and a half inches at 100 yards.
Mojo makes an apeture or peep sight and for the Enfield later on down the line you can check places like e bay for an A J Parker bolt on match rear sight.
The Mauser and Mosin are a little harder to scope unless you use a scout mount replacing the rear sight its either that or driill and tap the receiver.
www.brownells.com
There are a couple no drill mounts such as S & K and B square check e bay they sometimes turn up cheap.
Check into
www.surplusrifles.com for a few good articles.
I would go with either the Ishapore SMLE MK III 2A rifle or jungle carbine or check places like
www.auctionarms www.gunbroker.com www.gunsamerica.com for a good surplus Israeli 7.62 Mauser they turn up from time to time and are great shooters many with new barrels and little use.
the Mauser will not have a problem with any safe round and if in 7.62 NATO it should also handle the .308 Winchester as should the Ishapore MK III 2A since it uses newer steel and is not exactly the same as the older Enfield SMLE MK III actions.
Many however do worry about the difference in pressure between the factory 7.62 NATO and .308 Winchester rounds but with handloading though for me its not a big concern.
YMMV
Best of luck