Hi Alex,
I would recommend against the first option of building the board yourself. Even though I have a lot of experience even I ruined about 40% of all the TCBs I attempted to assemble, and that is a very costly loss rate.
Using the Mega as a base to DIY your own version is not a bad option but unless you add external components many features will not be available.
This thread which you have probably already seen has good information. Without any soldering at all, you can get read PPM or iBus receivers (SBus receivers would require additional components), and you can drive any serial motor controller (like the Sabertooth) or RC ESCs without problem. You can also connect the Benedini or Taigen sound card with no work, and if you have a recoil servo, run that too. If you want to turn on/off LEDs you can do that too without additional components (other than current limiting resistors which you need to use anyway), so long as you don't draw more then 20mA per output.
Beyond that you start to need additional components. LEDs with more than 20mA of current draw, the smoker, the high intensity flash, and IR functionality all require MOSFETs or transistors. Airsoft and mechanical recoil is not possible to get to work with the present firmware, because they use pins which the Mega boards do not bring out (even though the Mega uses the same processor as the TCB, it doesn't connect all the pins from the processor). There would of course be no on-board motor driver for the turret rotation and barrel elevation, it would be best to use a cheap RC speed controller for those (since they are such small motors you can easily get away with some cheap ones from China for a few dollars). Of course at this point you start to create quite the wiring mess.
Here is another very fascinating thread by Sergey who took the DIY approach to the extreme. It is in Russian but Google translate works very well.
If you just want to drive your tank around, have sound and a servo recoil, then it is pretty straightforward. If you want IR and lots of external gadgets, or need Airsoft, then it is not really going to be easy or work very well.
I suppose at this point in the project what is maybe needed is a simple shield with through-hole components only that would be easy for lay-people to assemble,
such as Rebus started.
I can't say when or if I would have time to work on such a thing, though I think it would be a worthy endeavor. In the meantime the situation you face is as I've described.
Another option is to post on RCU or other tank forums to see if anyone wants to sell an existing TCB. It's hard to believe, but not everybody liked it!
I'm sure they would be happy to make some money.