drtse, i have reskinned and reworked and played plenty of ghanaian djembes. i have never owned one, though, because i have not found any that come close to the sound i want. some have a really nice bass, but the tone and slap are quite important in this music and when you're a newbie you wouldn't have developed the technique or the ears to be able to properly appreciate the difference between different djembes.
you are right in saying that the type of wood found in ghana is generally very different to those used in guinea, and yes, the majority of ghanaian djembes i've seen were made from tweneboa wood, which is way softer than the main woods used to make drums in guinea and mali (if you press your fingernail into a tweneboa djembe, you will easily be able to make an indentation!). this also affects the sound quite a bit.
you can only inspect the inner bowl of a drum with the skin off the drum. if the store owner can let you see some drums that are unskinned, you will be able to inspect them, and if you end up choosing one, then he can skin it up for you. all the ghanaian djembes i've seen have been carved quite differently (and more roughly) in the inside than the others. also, the shape and proportions between stem and bowl are quite different. ghanaian djembes tend to have long stems. this is what gives their bass a bit of an edge, but to the detriment of the other tones.
note that ghanaian and north american goat skins produce completely different sounds than the ones from guinea, mali, ivory coast, etc.. djembe players tend to prefer skins coming from these countries and generally don't like the ghanaian skins (they tend to be very thin and weak) or the north american ones (once you are accustomed to the sound of a good malian or guinean skin, the north american ones will sound dead in comparison).
just because a djembe is made in guinea or mali doesn't necessarily make it a better djembe. there are plenty of 'hang on the wall' tourist type djembes coming from those countries too. be prepared to pay quite a bit more for a good guinea/mali djembe than you would one from ghana. whether it's worth the extra money will have something to do with your goals. you may find that your goals change over time. try a good guinean or malian djembe for comparison before you make a decision.
take dugafola's advice and get in touch with mohamed diaby. he's a very respected guinean percussionist who played with the famous group 'les merveilles de guinee'. his website is:
http://fabeproductions.comgood luck!