alloys buckle if they hit things. This leads to lack of balance and air loss, which may be very slow, or catastrophic. The wider they are, the more easily, because the inner edge of the "barrel" is further away from the spokes which are on the outer edge. If it's an expensive wheel it may be worth straightening.
It seems to me that the aftermarket wheels sold to boy racers on price have less metal in them than the ones supplied by car manufacturers. You may notice the spokes are thinner and/or further apart and less of them. I don't think I've ever had a steel wheel go wrong, but alloys, yes.
You can get a set from scrappers cheaply enough, easily matched if they are manufacturer's originals, but it would be sensible to have them inspected and perhaps refurbed by an alloy wheel specialist.
Split rims (tiny bolts hold them together) need to be dismantled and rebuilt with skill, care and new seals. A bodge job may not be apparent but could be disastrous.
The wheel nuts used with steel wheels do not fit alloy wheels correctly and may crack them, so you need a new set.
I prefer a full-size spare. but very big alloys may not fit in the well in the boot.
