2 Core Sets is where I would start. It'll set you back $60-80, but since you're coming from GW gaming, this is going to be a drop in the bucket. I've been there myself, so I feel your pain.
2 Core Sets gives you 2 X-Wings and 4 TIE Fighters, as well as an extra set of templates and dice. The dice are key, because you only get 3 of each in the Core Set, which is pretty silly since you'll need a minimum of 4 of each to play the game smoothly. Why?
An X-Wing rolls 3 attack dice normally, unless you're at Range 1, then it needs to roll a 4th. A TIE Fighter rolls 3 dice to defend, unless it's at Range 3, then it rolls 4. It could be rolling 5 if an asteroid is in the way. So really, you don't want the added book keeping of remembering die results while you roll other dice again to add them up.
The extra templates also make playing with your opponent easier, as you don't have to pass things back and forth.
Now, on to the models; the single Core experience is great for a demo game, and that's about it. It's a pretty one-sided affair. If the X-Wing can take out one of the TIE's early, Rebels win, otherwise the Imperials will likely close it out because they can box your lone fight in a lot easier. You'll grow really tired of that matchup soon enough.
2 X-Wings vs 4 TIE Fighters gives you some room to play with, using the different pilots and upgrades, and match up the points values on both sides. As a GW player, this should all make sense and be familiar to you.
Beyond that, just expand the game to your personal tastes. If you've got some organized play happening near you, read strategy threads to inform your buying decisions, otherwise just buy what appeals to you, model-wise. The only caveat is that each ship pack has a specific set of upgrade cards that can be used by other ships. For example, the A-Wing comes with "Push the Limit" which melts faces when used by two of the TIE Interceptor pilots, but oddly does not come in their packs.
So if you wanted to do a popular and effective TIE Interceptor build, you would need to not only buy the TIE Interceptor packs, but a couple of A-Wing packs as well, for the cards. For casual play, just proxy it, but again, if you're playing in a league or a tournament, you'll need the official components. Cost-wise, I think a fully fleshed out X-Wing model collection, Consisting of 2 Cores, and 2 X-Wings/Firesprays/YT-1300's and 4 TIE Fighters/TIE Advanced/A-Wings/Y-Wings/TIE Interceptors will still come in way less than a single Warhammer army, BUT you don't have to invest in the painting/modelling, AND you have two fully playable "armies" to work with, including a lot of variety in your list building.
So back to my original point; buy 2 Cores, and if you really love it, just take your time expanding out from there. If you have a regular opponent, you can split the costs if you both want to claim a side. Even if the cost-splitting isn't happening, if you can play regularly, you'll have a blast and it'll pay for itself in terms of money vs time in entertainment dollars.