M1 or M2 are basically the same.
I use the Nordic Comp bolt release it works well and won't break the bank.
Mag ext tube would be a +7 for a 28" bbl gun. The +7 ext tube is is 17.63" long so it would not be exactly flush with the end of the bbl just a little short. A +8 is 20.28" long and would stick out past the end of the bbl a little. (measured from the end of the forearm) Give me a call or send me an email if I can help.
There are several different bolt release buttons on the market.
Briley does have one available for the Benelli cost $45.00 has to be drilled and tapped etc.
Nordic makes one for about $15.00 (Alum) your existing bolt release must be drilled and tapped.
Arredondo makes one for about $20.00 (plastic) again your existing bolt release must be drilled and tapped.
GG&G includes a drilled and tapped bolt catch about $100.00
Progressive machine (alum) goes on with two-sided tape. (not sure they and still in business but shooters connections shows they still have some in stock) $44.00
DMW is a metal bar/plate that is retained by the forend and covers the bolt release button.
I have installed them all and they all work. Some I like better that others but that is a personal choice.
There are probably several more on the market or you could always make your own.
Good luck
WTS a Nordic Components Gift Certificate good for one of each item: mag tube ext, bbl clamp and follower.
It is good for and model they offer. See copy of cert below
$100.00
Contact me via email at jeff at c-rums dot com
You can also trim out the notch at about a 45 deg angle to the rear of the bolt it is faster and easier than epoxy and you don't have to worry about the epoxy coming out.
I would say it is a pretty fair deal. Check some of the auction sites and see what used M1's are going for I think you will find that is what quite a few used M1's are selling for.
Good luck