If you are sticking to mainly outdoor play, the $100 you were going to spend on the pistol might be better spent on your optic and / or a bipod. Pistols in the great outdoors don't get used much, and even in indoor games get used mainly to screw around with when you are bored with running your primary. Well, and to fight to someplace where you can unf**k your primary when it quits.
Fair warning: It has been a few years since I have played outdoors, and even longer since I've done DMR work (long enough that we were Squad Designated Marksmen rather than DMs, and my "DMR" was an accuratized M-4 with an ACOG, bipod and suppressor with heavier ammo). So, the community understanding of what a DM does today may have shifted from what I believe the role of the DM to be. In the hoary ancient days, the job of the SDM was to play guardian angel rather than door-kicker. We moved out with the rest of the squad until one of two things happened: 1) If we took fire, we moved to a stable shooting position and engaged the enemy with accurate fire at range while the rest of the element closed with and destroyed the enemy or 2) when we reached our objective, the SDM sought elevation and clear field of fire to provide intel on the target and engage threats while the rest of the element moved in and secured the objective.
Looking at the role as posited above, I'd say that the key priorities would be:
0) Decent eye protection. And whatever face protection you want / need / like.
1) Get an accurate primary.
2) Pick up a decent pair of boots. Although, arguably, this could be priority one if you can borrow an AEG from somebody.
3) Get a decent VARIABLE magnification optic. Failing that, get a 4x or lower fixed optic. Variable is very good, since you can see trouble coming and communicate it to your team before you are in the engagement envelope, but can also turn it down to a useful magnification for engaging threats. I ran a $50 Tasco 3-9x scope for a couple of years and it worked well enough that I rarely swapped it out for the ACOG.
4) Gloves, camo uniform(s) of choice.
5) Spare mags and batteries as needed.
6) Load carriage equipment (plate carrier / chest rig / battle belt).
7) Hydration pack / camelback / Nalgene bottles / canteens.
Second primary for when the first one quits. Bonus points if it uses same mags / batteries and other primary.
9) Comms gear.
10) Grenade launcher / grenades / mines / RPG.
11) New Car.
12) Girlfriend / prostitute / Realdoll.
13) Steak dinner for everyone you know.
14) A secondary (pistol) that ISN'T an M9 derivative or made by WE.
15) Herpes.
16) An M9.
17) Anything made by WE.
Specific to that pistol (and leaving aside that I, personally, loathe both WE and the M9 platform), the rail on it is outside the dimensions of the actual Beretta 92FS / M9. So, it may not fit into some of the more high-speed hard-shell holsters (like the Blackhawk Serpa series), and some soft-sided holsters tend to drop the mags out of M9s. If you care about being able to carry in a hard-shelled holster, consider anything Glock, HK USP / USP compact, HK45, or SIG P220, P226.