You can convert...
You can convert your stock alternator to external regulation. I have owned three of the M-25's in different boats and the max current I ever used is 90 amp due to the belt size.
Read the article bellow and it will show how to convert to a 90 amp alternator on an M-25 for a lot less money than a Balmar.
If I was to do it again, and I had AGM's, I'd simply buy an external regulation plate from Leece-Neville through Ron's Auto Electric and upgrade my stock alternator and be done with it. You will also want to use alt & batt temp limiting and current limiting with AGM's.
If you want a bigger alternator (read not really necessary in many instances) the Leece-Neville's stock regulator is adjustable so you don't fry your batteries by over charging them.
I've had two Balmar external regulators fail, older ARS regs but since those not a single one. The stock adjustable regulator that comes standard on the Leece-Neville is a simple little regulator and it works ok.
Our current boat spent five years as a full time live aboard, on the hook, cruising vessel using her stock 55 amp Westerbeke alternator and one 80 watt solar panel.
After much experience, with many alternator upgrades, I have discovered that it's mostly a very successful "marketing gimmick" and in a number of situations, but not all, and much of the time, these large alternators and fancy multi-stage regulators are not totally necessary for a weekend warrior.
I've physically compared a Balmar multi-stage regulator to the Leece-Neville pictured bellow, on the same alternator, on the same boat, and at a 50% discharge the difference getting back to 90% was about four to six minutes.. They both charged at the same rate. The only reason I did this comparison is because my ARS-4 failed and I had to swap out, and convert back, to the stock Leece-Neville regulator while I was patiently waiting for the Balmar replacement. I was curious if there was a difference so I measured it. I don't know to many other folks who have actually physically compared a "dumb regulator" to a "smart regulator", using the same alternator, to even know the difference or lack there of.
P.S. You can also buy the exact voltage regulator, pictured bellow, from Leece-Neville and bolt it to your existing alternator, and adjust the max output to match your AGM's, for pennies of what it will cost you to go to a Balmar.. Just know that if you run long enough your AGM's should eventually float and this reg won't do that.
Universal Diesel M-25 Alternator Upgrade
Leece-Neville adjustable voltage regulator: