You know a good thing about levelling Sharpshooter first?
You gain skills with the Xbow, such as calculating the trajectory of the bolt.
When you apply this to Medic, you must learn to treat the dart as a Xbow bolt.
Therefore, the MP7's "dodgyness" is almost cut in half.
And as to a more powerful weapon, how much more powerful can you get than the Xbow? it can oneshot a scrake, and twoshot a fleshpound on suicidal. This was on, i think, a 5-man game with the last medic soloing the wave.
If you want an assault rifle-style weapon, the L22 bullpup, or AK47 fits the bill.
If you feel like levelling zerker, Katana it is then.
TL;DR there's no need for another offensive weapon. Medics shouldn't be on the offence all the time.