Glad to hear you are only a noob to airsoft. Welcome to airsoft and to the forums. I raised an eyebrow to the 300' question and automatically assumed you might still be dreaming of the day your mom won't have to pick you up from the mall. Sorry. Its not that I dislike our teenage couterparts but they tend to post hopes and dreams instead of actual goals. With that said you have nice tastes in replicas.

Don't forget about a sidearm. Regardless of what anyone says Tokyo Marui GBB pistols are some of the most skirmishable handgun replicas on the market.
Don't go cheap on your air rig. This is your primary air rig and you don't need a carbonfiber wraped tank. Go buy a used one in person at a paintball park. BUT CHECK THE HYDRO DATE!, no one fills an out of date tank. Look specifically for a low output or 450PSI output tank if they have it. The links below are great sorces from guys that rely on their rigs. I would just buy one if I were you unless you have some of the parts already.
http://classicairsoft.org/forum/showthread.php?8764-Tumerboy-s-rig-guide
http://classicairsoft.org/forum/showthread.php?9002-WTS-Pre-assembled-dual-reg-gas-rigs
HPA is not harder on your gun! Its totaly different from using in-gas mags. In-gas mags require enough gas to opperate a whole mag plus while HPA does not. Which is why your first shots are terribely hot or low. Hot, due to excess pressure or low due to too much pressure in the mag. HPA opperates on maintaining a PSI level and topping off the mag as the volume and pressure changes. Now, its not harder on your gun but it may be harder on your hop-up bucking. I have exceed 800FPS with external air and a rifle before. Your mag on HPA is just a cylinder or an expansion chamber with CO2. CO2 mags opperate on 12g which have MUCH higher PSI, >1000PSI. Unregulated anything, be it propane or CO2, opperates in a balence of pressure and how long the valve in knocked open by the striker so it will always change. HPA, regarless of how long the valve is open will only allow "said" PSI. Volume does matter in this case but you are essentially looking at the difference of nimh and 7.4v lipos in terms of performance and wear and tear on motors if that makes any sence.
You do not need to upgrade a thing to go external air. I run my TM 1911 on higher PSI than any of my other replicas, ~140+PSI. All you need to focus on is tapping that mag, your air rig and accuracy upgrades as said above. DO not store your replicas outside your closet. You don't want to lose a seal.
You might want to try placing your tank in a camlebak or a hydrant pouch off of country comm. I have tried many things buy since you are only running a bolt action rifle try 13ci 3000psi tanks. They are very slim and should be very easy to hide.
I used 1/8"NPT tap which requires a 11/32" bit and a pilot hole. Go easy on the tap. Cheak youtube on how to properly tap a hole. Aluminum is very easy so you have that going for you. Just take is slow and if you have it a 3-1 oil would not hurt. Of course you'll need to take the mag completely apart and clean it afterward. I put mine center of the semi-circle of the magwell on the left side of the mag. Looks good too, and it does not interfer with the folding of the stock.
Good luck and let us know how the the air rig goes,
mateba