Best way to change to Xenons is to find someone on here breaking a 172 on here and buy the complete setup.
As for springs, eibach proline are very good at handling and comfort but don't lower more than 5mm. Cooksports are good at handling but a bit more crashy (still comfortable), lower by 25mm. Then Spax these are firmer than the other two so not as comfort wise, however I thought they handled well because of the firmness. Btw i have only said these three as I have personally had them on my cup.
Exhausts, ktec and yozza seem to be the most popular on 172's.
Best thing you can do is pay the £12 membership as these thing come up for sale on here, plus the advice/guides have saved me much more than the membership costs.
Btw I have the spax springs and buckets for sale