Congrats, I play a 615 and love it. But whatever you buy get it set up professionally, it is amazing the difference that can make.
To your first question Capo’s. If you go to mandolin cafe you will see that capos are very controversial, along with the topics Thile v. Monroe, reading notes. v. Tab, or are new gibsons over priced. IMHO these are mostly subjective or really need the answer to be “it depends” .
CAPOS: Mandolins with their uniform string intervals ( I.e. no b string that mixes things up) usually don’t need capos in jams since all you do is move your hands up or down for chords or scales and they are the same. Of course I speak of closed position scales and chop chords. That said capos are seen by many in the mando community as crutches and not kosher. Truth be told open chords sound different than closed position chords, and if you need a A# open chord sound by all means use them. Irish and British mando players use capos a lot but they play mandolas and octave mandolins more and have more space on the neck to use them. Personally, the mando is so small capos usually cramps me, but I have used them in Live situations which flakey singers. ( I now think Ben’s next lesson should closed position scales…if the interweb police are listening or watching me through my tv)
NEW STRINGS: Probably not, Eastman puts J74s on there and they should last you a bit. When you change strings know that the bridge is not attached ( like a violin) So change one string at a time. That said I love Elixirs not for their better sound than J74s but longevity. So ride out the 74s, put on Elixirs for a longer interval between changes.