Yes, you can install the T-70 and other X-Wing types now with working S-Foils, and still have the basic T65 available thanks to Jeremy's S-foil Hook. You can also install any other ship into empty slots, or overwrite existing ships using BHEdit, or MXvTED (I prefer the latter myself).
It isn't that difficult to do, but for somebody not familiar with installing mods manually in general you're going to want to download the editors and read the tutorials found on Darksaber's website.
http://www.darksaber.xwaupgrade.com
Also, if you want a shiplist picture of the ship you're going to need DatTech, or Jeremy's new tool to create a new dat file with the stuff you want to show up.
There is a learning curve for all this, but it's not very hard comparatively to some other games out there with how old this game is.
Be warned though, if you change certain things and try to play multiplayer with someone without the exact same changes, you will either hard crash (in the event you're trying to use a mod ship the other person doesn't have (this would also apply if it was in a different slot in their game) or major latency issues in the event that you're using the Imperial Concourse and they're using the Rebel Concourse (due to the major changes in the strings.txt file, that will make the game unplayable due to the resulting lag. I've written up some documentation about this a few years ago, but I need to find where I put it before I can share it.
As for what kind of mission to do, if you're wanting to do a LP and don't want to make a mission from scratch, then you might just want to play the first mission regarding joining the Alliance in XWA,it's 10BM8W.TIE. For the A-Wing I'd recommend 1B1M3G.TIE, the third mission after completing the two recruitment missions.
Otherwise I have a mission I created recently based off of the FFG X-Wing fan campaign Heroes of the Aturi Cluster first mission, with a bit of creative license used to adapt the table top game to a digital narrative format, I just need to finish the animated briefing and do some ingame messages, perhaps some VO work and it'll be ready, but otherwise it's playable. You'll be able to choose either X or Y wing at first to fly it (though it could be adapted to utilize any ship), but the intention of HOTAC is to allow upgrade to new ships later as pilot skill increases.
It's not very difficult, per se if you think fast and are paying attention to your surroundings, but you'll definitively fail if you aren't familiar with XWA game mechanics and paying attention to the situation as it unfolds.
It can be played in singleplayer, but it is also intended to be able to be adapted to and played using the XWA COOP campaign mod, the balance with multiple players has not been tested, but it can be completed with one player either way.
XWAUCP with appropriate hooks from Jeremy will be needed to play it due to the direction I intend to take things in the future. And in the future missions specific mods being installed in specific slots may be required.
If interested in testing it multiplayer let me know and we can work out some contact info and try to set up a game to test the multiplayer balance. Otherwise I can finish stuff up and you can critique it for me solo. Nobody else has yet played it, so feedback would be nice in regards to difficulty.