+1 to what Airsoftmaniacman posted. I just have a little more to add in response to your questions...
1. Are HPA guns worth the money? I know "worth the money" is a relative term. Will a HPA system perform better than an AEG after investing the same about of money in both builds?
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You can make an AEG perform nearly or as well as HPA powered guns in some respects. The biggest difference is in barrel volume. With an AEG your barrel volume is limited to what air is contained in the cylinder between the piston head when retracted and the cylinder head (assuming a type 0 cylinder). With HPA systems, you can match volume to any barrel length. In DMR's, that makes a hugh difference. The other issue is that since AEGs are mechanical in nature, they have a higher likelihood of breaking down ... especially in a high stress build. Most of the time, if you have a failure in an HPA system, it can be fixed by reseting the FCU.
One of the reasons why I switched to HPA is because I always seemed to be teching AEGs. Teching is fine when you have the time to do it but it is a pain when you are doing AEG repairs to have a gun to field the next day. Now I can work on AEGs when I want to ...not when I have to. In the long run, you will likely save money on AEG parts when you go HPA.
2. Full engine or cylinder replacement? For a basic field or CQB rifle are the upgrade options offered on an engine really needed over the less expensive drop in cylinder?
I have a P* fusion engine and have been very happey with it. I also have several team members that decided to go with the Wolverine SMP or Hydra. The P* fusion engine has the advantage of having detailed settings that can be finely tweaked. It takes a little time but usually the results are worth it. If you don't want to spend a lot of time tweaking your systems, go with a cylinder replacement HPA system. Most have settings only for rate of fire and poppet dwell (FPS adjustment). The other advantage of a cylinder replacement system is that it can be installed in more gun bodies. The P* star V2 fusion engine seems to prefer VFC M4 bodies. If you have a more exotic airsoft gun, you may have to modify it for the FE to fit in it.
Go with the SMP. It is tested. The initial reviews of the Jack have been promising but I have heard from retailers that the smaller solenoid on the Jack is more susceptible to being fouled with dirt and grit. If your gun requires an offset nozzle, get the Wolverine Hydra instead. You also might want to take a look at the Wolverine Inferno as well. It comes at a cheaper price point but has the same internal structure as the Hydra. You can later upgrade it to Hydra standards if you like.
6. How long will a tank last me? On average, with a common HPA rifle set up, how long should I expect a singe tank to last? Is it normal to need a second tank for full day of play?
That very much depends on your setup. From what I have seen, Wolverine products seem to be pretty air efficient. If you decide to do an air efficiency P* build you can go a long way on one tank. I am running my P* at 55PSI to get 325 fps with .30g bbs (1.55J). I don't spam the trigger (I use 3 round burst) but I do shoot quite a bit during a gameday. With those settings, I typically go through @1500 PSI per an 8 hour game day. At that great shoot fest known as Fulda Gap, I emptied my 90 CI/4500 PSI tank after shooting approximately 8000 rounds. (it was a target rich environment). Hopefully, that will give you an idea how many rounds you can get out of a tank.
I hope this info helps. PM me if you have any other questions.