get 2 core sets 1st. Then decide which side you want to play
get x2 Xwings x2 Ywings. Both comes with unique upgrades and pilots you would really want to use, eg. R2 astromech from Ywing is great when paired up with expert handling from Xwing and R5 astromech from Xwings help the Ywings ward of critical effects, and the Ywing also has unique astromechs that are pretty awesome on either ship. I recommend 2 Ywing although honestly only 1 is needed. you may or may not want to buy 1 TIEadvanced just for the squad leader skill upgrade, depending on your budget.
get 1 more core set, then get 1x TIE 1x TIE adv. This will give you everything you need for the lowest cost. Then depending on your playstyle, you could opt for either one more TIE to go the full 8 TIE build, or another TIE adv for builds more similar in style to the rebels
get 3 core sets in total, get 1 of each expansion. you should now have everything you need. then buy specific stuff for specific needs, eg. additional Y wing or TIE to up the numbers, maybe 1 more TIE adv so both sides can use squad leader etc
EDIT: sure is irritating that i cant post on this forum with my phone