Girl...
Clear & simple:
1. The Timbo track is already mastered.
2. If you drop vox on it, all you need is (probably) good processing on your vocals & a good mix (the "mix" being mixing your vocals in well with the track). That should be done by the same person who records your vocals to the track. If it's you, do your best & go-for-broke.
3. If I put original tracks on my mixtapes, they are mastered before they make it to the mixtape, but that's cause they gotta be mastered before they are ready for public consumption.
3a. Altruist was still kinda right with his statement - you could probably get away with a good 2-track mix, unmastered, on your mixtape (of original stuff, I mean). If it's your vox over a beat offa wax or whatever, then leave it alone, especially if it's sounding hella good as an instrumental. So, if economics is an issue, or you're just happy with the sound, GO FOR IT!!!
4. Do a search online & read up on what mastering is, when & why it's done.
5. Do a search online & read up on mixing techinques & getting a good 2-track mix.
6. Fuck all that other shit. Trust me...
Hope that helps...
Diendolo