Hi Sat

To try and answer your questions....
1. Technically we are all capable of continuous improvement with practice (until such things as age start to take their toll) so you can work with poor triggers etc. its just that improvement process will be much harder and take much longer
2. Modern pellets are very, very good and cheaper pellets such as RWS Geco are certainly capable of putting 5 shots in the 10 ring through my Morini when rested, these pellets are very popular at our club - however the pellets you mention are no doubt just as capable and certainly won't be hampering your scores IMO.
3. As you say you are going to be taking this seriously - then from my own experience I would definately put the money toward a S/H Steyr/Morini/FWB from the off and this very question has been covered both here.... http://www.airgunbbs.com/showthread....sters-any-good
My own thoughts are you'll quickly want something much better than the Rohm's trigger and then take a hit when you come to sell the Rohm.
4. see answer 1

It looks as though you have joined a club already so perhaps you can have a chat with a few others there and try some other guns, all the people I know are more than happy to let new shooters try their own guns and offer their own opinions both good and bad of their pistols.
For other opinions of other seasoned target shooters can I suggest you ask the same or a similar question in the Target shooting section of this BBS or a more focused forum such as targettalk.org