G17 is perfectly good gun for competition. G34 will need same upgrades as G17, only difference G34 has longer slide hence longer sight radius.
If I were you and just starting in uspsa/idpa, for starters I would only change out sights. You can add trigger kit, stippling and other mods later if you want.
Once you get more involved, you meet new people and most of us don't mind letting other shooters try out equipment. That way you can see what you like better. From personal experience I can say that equipment contributes probably 30% of you performance, rest 70% is skill. A lot of new competitors tend to get hang up on equipment ( me used to be one of them). Spend your money where it counts - ammo, training, matches.