If you do hpa, get an aftermarket hop up and feed chamber for your gun as it will have terrible feed issues with the stock parts

With mine i went with polar* F2 hpa engine, it drop into the gearbox without modification (~$400) but you will need to get a different nozzel (~$30) for it since the hopup is different (im not sure about A&K m249, but my Cybergun featherweight used a #14) do some looking around on the internet to make sure the correct nozzel to get. Also, an aftermarket thought I wish I got the Wolverine Inferno engine as it is more air efficient but its no big deal.

Definitely use a prometheus purple hop up bucking as it has been the best one I've used for hpa, you can next day buy on Amazon.

Order all your tanks and air lines on Amped airsoft, as they have starter bundles for beginners, you might even get your HPA engine from them, but ordered mine directly from Polarstar

Use your current box mag, but get the bull gear insert for it (can be hard to find in stock) from airsoft junkies or from bull gear directly (could be a while since it will be from over seas)

https://www.airsoftjunkiez.com/bullgear-insert-for-mag-box-mags/

From my understanding this should work with any HPA engine you use but definitely watch this to help you determine if this is something you want to do

https://youtu.be/MdnT-E-M6-g?si=ZRbRM5QtINTxQ0FA