Having replaced many an airbag, here's what I can tell you: It's not hard, but it isn't fun either.
Are you sure that those are the only two things that deployed? Before I had a scan tool, I spent many hours trying to figure out why the airbag light didn't go out when it was a stupid pretensioner that didn't even look blown. I'd recommend buying a scan tool that can talk to the airbag module, which should cost about $100. If you don't think you'll ever need a scan tool again, find someone with a scan tool that can talk to the airbag module and beg for help. It will be worth it to be sure of what you need.
The curtain airbag (when not deployed) looks like a long sausage attached to several mounting brackets that are bolted to the roof structure. There's a harness connector over the rear door that connects to the explosive charge that inflates the airbag. All that is covered up by the headliner, which is then held in place by the A, B, and C pillar plastic trim pieces and the sun visors and grab handles.
The sequence is to disconnect the battery, then remove the trim pieces, visors, and grab handles. Watch videos on youtube on how to get the trim pieces off without breaking them. Then pull the headliner down a few inches, enough to get at the airbag bracket bolts (be careful not to add any more creases to the headliner, they don't ever disappear once created). Unplug the harness connector, which has a little plastic locking clip you don't want to break (more youtube videos). Put in the new airbag, bolt it down, and attach the harness connector. Don't forget to snap down the locking clip. Before you put the headliner and trim back, you're going to want to test it, but since you have multiple bags to replace, do the rest of the repairing before you put the trim back.
For the seat, just go to a junkyard and buy a used seat. Sure you can replace the bag and sew up the seat, but it will take you hours and if you use the wrong thread the seam will either fail prematurely, or fail to rip in an accident and kill you. Just replace the whole seat. Again, disconnect the battery before disconnecting anything. Youtube videos will show you how to replace the seat.
Once everything is connected, reconnect the battery and start the car. The airbag light should go out after 5 seconds, but wait at least 15-20 seconds before celebrating. Sometimes they come back on. If it does, go back to the scan tool to see what the problem is. Once the light is out and stays out, then put all the trim back (been there, did it twice, didn't like it).
That's why I suggested you need a scan tool that can talk to the airbag module, because you don't know what elements need replacing. Some seatbelts have the pretensioner built into the belt reel, some have it built into the buckle. The accident might also have damaged wiring for the sensors. You just don't know what parts of the SRS system need fixing without a scan tool. With the cost of airbag parts, you don't want to be guessing, let alone the time you will waste chasing your ass. An Autel (or other decent brand) OBD/SRS/ABS scan tool is the best $100 you can spend on this project car.
If the seatbelt is locked, then the SRS charge inside it has blown. There are places that will repair it for you (you remove it, send it to them, they fix it and send it back, you reinstall), but generally I've found that it's cheaper/faster to get a used/un-deployed one off ebay than to get yours repaired. You will also want to check the buckle side to see if it has an airbag charge on it (if there's a tube that runs parallel to the seat rail that's attached to the buckle down near where the buckle bolts to the seat, it has a pretensioner built into it) and you might as well replace it too. They are really hard to tell visually if they've blown, and without a scan tool to positively identify it as a fault, it's easier to just replace it.
As for the airbag module, it depends. I've had modules that intentionally lock/damage themselves in an accident and have to be repaired or replaced, while others I've just had to fix the bags and the light went out. Not sure how yours might work (other experts feel free to chime in), but if the scan tool can't even talk to the module, that's a definite clue. If you have to send it out, I recommend MyAirBags.com, they have fast turnaround and I've never had a problem, though there are cheaper alternatives. One more caution on the airbag modules: most are programmed for the VIN of the car. You can't just get one from a junkyard and put it in your car without getting it reprogramed by a dealer. From what I've been told, they are reluctant to do that service because of the liability if the SRS system fails, they will want to sell you a new one. So if the module isn't working, try sending it out for repair.
If you're able to do the job yourself, by all means. I think it's a great idea. Just know that this kind of work isn't easy and there's a lot to learn. As RocketmanKarl is saying, this isn't like changing your oil. Your safety is in question here and the job must be done absolutely correct the first time or you shouldn't be doing it.
This is why shops charge such a high price for airbag work. This is also why if your airbags deploy usually the insurance company will just write it off.
So in short take your time and make an informed/intelligent decision. I have done a lot of car repair in my time including body work and I'd be thinking twice before doing airbag replacement or at least doing a pile of research.