Depends on what you need or want. I have the INVG mount, ROCK SOLID!!! It's bulky looking, but makes up for it in that it's far lighter than it looks and it flips straight up instead of back, and can fold to the side out of the way, mitigating bumps on walls and doorways when not in use. 100% range of adj., never had it that good with other gear. Flip down is only 90deg., not 120 or so, so it's faster to deploy. No need for bungees with the INVG (but using them is a redundancy I can't escape) it does away with the J-arm completely (I hear you, I hate that goddamn thing) and uses a dual dovetail, which you can remove the "dual" portion if using on one eye only lending to more use of your night eye. I got the INVG based on using GI shit, and I'll never go back. Wilcox makes some nice, compact light ones, but you gotta use their $200 dovetail J-arm. At the end of the day, all the quality mounts will run you about the same, it just depends on the attributes you want out of your gear.
As for the helmet, you got an Airsoft helmet so I'm assuming you don't need ballistic protection? I got the Crye Airframe because it was best blend for buck: 3A armor but with carbon fiber weight (it's a pound less than the rest and has ventilation). It's nice, but costly. When Vic at TNVC showed me that damn Crye soft cover, I was sold. $100 for it, $50 for the Wilcox L4 plastic shroud (works great on that soft cover with the INVG, btw). Now I seldom use the helmet. This isn't a skullcrusher btw! It's like wearing a baseball cap but instead of a bill, you have NODs that flip down. Nice. Goes on like a ball cap with a chin strap. VERY comfortable, more so than my nice helmet!
My suggestion: get a good mount like the INVG and dovetail (or whatever floats your boat, but know dovetail mounting in general are superior to bayonet, but you're gonna pay, and the Wilcox J-arms do cost more than the dual dovetail does and it does flex more). Then get the Crye soft cover and that L4 shroud. The shroud comes with some Wilcox sets.
If you NEED or want a helmet later, get a high or mid cut ACH, that's my honest opinion. You'll be able to use ear-pro with it and it'll cost half what a carbon fiber Crye helmet does. Considering you'll use the soft cover 99% of the time, this is the way to go, but you might not notice that until you actually try out the soft cover. I'm not embellishing on this, it's a nice piece of gear.
If you get a helmet that ISN'T a mil helmet or provides 3A protection, you should consider one of the finer bump helmets that use the same suspension as the combat helmets. They'll be much more stable and reduce wobble. The Wendy and others are popular, but damn, they're only a few hundred short of having full ballistic protection, so it's up to you.
This is just my experience as someone who bought all this shit last year. Also, TNVC is a GREAT place to do business with, they've really taken care of me. Can't speak highly enough of them and their staff.
Biggest advice I can give you is don't skimp on a mount. You're already buying another, do you want to find you don't like your next choice that saved you a hundred or so, only to buy the $520 mount later anyway? So good mount, get the soft cover, trust me you'll get good use out of it and it'll save you money interim if you do in fact need a helmet. If you don't you may find the soft cover is all you need, thus saving hundreds more (or covering the mount altogether, including the soft cover and shroud).
