420 sounds about the going price for it.
Stock hopup rubber... I don't really have any problems with it... See how it shoots first than decide. If it ain't broke, don't fix it.
Mosfets are manadatory for 11.1v LiPos, aren't really necessary for 7.4v but if you're planning on going to 11.1v, get a mosfet.
Most brand batteries are alright.
PMAGs are nice if you can afford them, but as BioRage can attest to, you might cringe everytime you drop one for a tactical reload. KA midcaps should work fine (I don't have any experience with them, but heard they work). I use Beta Project PMAGs and PTS PMAGs and the Beta Project ones wobble and quality can't compare with the PTS. Just PTS is very expensive for AEG mags.
I'm in the same situation with the goggles. I wear paintball goggles because I go to paintball fields a lot to play and they don't allow ballistic. ESS Striker series is the way to go though for ballistic goggles with glasses. Make sure you get real ones and not clones.
Rigs are personal preference. Some like battle belts more, others like the full plate carriers, others like tactical vests, while some like a high-speed low-drag minimalist chest rig style. All up to you.
For something as small as a Defender, a 1-point sling would probably be the best, but make sure the rig you get has a way of getting the sling off the back of your neck. Zipties work great for this.
The above is just my opinion; take it as a grain of salt.
Hope you gear up soon and have some fun.
Primary: Custom ZTW VFC AR - Custom RTW VFC AR - KJW KC02
Secondary: Custom TM PX4 - KJW Mk1 - KJW M9
Camo: Pencott Greenzone - Multicam - Ranger Green - MARPAT