Specific to the Tanaka:
The M700's (including AICS) and M40's are pretty much the same internally, it's just a matter of looks. They take the same mags (except the AICS can't use the short mags), and IIRC have the same barrel. If you plan on being active with it, get the long mags as they have a larger gas chamber. Don't be tempted by the KJW M700 mags as they are far inferior quality. Their impact plate wears out way faster and they are more prone to feeding issues. In addition, the extra $20 for the long mags gets you almost 3 times the ammo capacity (29rds vs 10rds). I personally wouldn't recommend the AICS simply because it weighs like 3 times as much as the regular M700 but if you have to have the looks, then go for it. King Arms makes a conversion kit for them so you can use VSR-10 inner barrels. IF you can find it, get the G&G rubber kit for the gun.
Parts wise, your only option is Hong Kong. Some of the parts were hard to come by 3 years ago, now isn't any better. G&G and Best Gun parts are all but gone, King Arms is your only option for most parts. Be prepared to be looking a while for the more uncommon parts. WGC and Den Trinity are going to be your best bets. Ehobby and Redwolf don't stock much along this lines. Evike supposedly has some of the parts, but I wouldn't hold my breath on that being true. The King Arms upgrade nozzle costs $15, get it with the gun, especially if you are getting a restricted one. Wait on the chamber until you see how the gun performs stock (with the nozzle). Do not get the Power Pack or Power Bolts (G&G or King Arms) as they are completely unnecessary. Do get some of the striker plate and spring kits (I've only used the G&G, KA wasn't making them at the time) as they are good replacements for when your stock ones wear out. If you don't get one of the conversion chambers, EDGI makes barrels pre cut for Tanaka.
For consistencies sake you could upgrade your mags to use HPA, but that's a lot of hassle and cost. It can't be beat performance wise, but I never found the pros to out weigh the cons. Use propane, not green gas as green gas over lubricates the gun.
Lastly, while the KJW M700 is mostly a clone of the Tanaka's, do not be tempted by it's price. It's stock performance doesn't even come close to touching the Tanaka's stock performance. The amount of money you will have to spend to get the KJW to keep up with a Tanaka is more than the cost of the Tanaka.
