Thinking of getting Pandemic...should I buy one or both of the expansion packs along with the base game?
Never played Pandemic solo so I've been hesitating waiting for a better answer, but as you have none...

I'd get the base game; you can always add later if you really enjoy it solo. Pandemic's base game is good anyway - it's not a game that needs its expansions.

If you're thinking of buying the set to save on future postage, then that's your call ;-)  It would not be a bad decision to do so.

Do buy the latest edition so any expansions you buy later match artistically.  

'The Brink' expansion's main feature is to be able to play with 5/6 players, less relevant to you, though you do need it for 'In The Lab' (which I've not played, so I'll stop there :-)