Hi mate, like Zitch said its an M-LOK rail section on the bottom. You've got two options in terms of foregrips, A) just use an m-lok compatible foregrip or B) get an adapter to convert it to RIS then get a "standard" foregrip
For BB Weight it depends on whether you play indoor/ outdoor (as heavier weights are less affected by factors such as wind - from my limited knowledge anyway). In terms of a speed loader any should work, i think they are standard M4 mags which should be ok. I am also a new player so someone with more experience may be able to offer more assistance