do it all yourslef mate it's not a difficult job the rear beam comes off pretty easily, most cars in scrap yards are front end smashes so a new one will be easy to get, might be worth getting a new / second hand rear damper too as yours may be bent / damaged but you should be able to get that with the beam no problem.
i'd buy a new alloy though as there may be hairline cracks in the one that got twatted which may have weakened it up and will only need a pothole to shatter it, (especially if it's a cheaper brand of alloy wheel)
dont forget once it's all on the road to have the alignment checked over shouldn't cost more than £30 to have 4 wheel alignment done.
if it's just the seat airbag get an upholsterer to sew it back up it's up to you if you get a new airbag put in but if you dont you'll need a resistor to fool the ecu into thinking it's got one in there.
shouldn't cost nay more than £500 -£600 to get it all on the road again which i bet is cheaper than your excess.
one word of warnign though get your car out of the recovery place asap as they charge mega money for insurance storage