Welcome to the site
To be honest mate. If your looking for a used
then Warranty won't be too much of an issue to you.
If you wish to tune then there is LOADS of places that you could get it done including Pumaspeed... and no the dealers don't like mods, from any place.
if you get a fast ford mag theres loads of places listed in there.
Good Luck with selling the Astra and looking for your next car... the
wouldn't let you down; running, performance or looks... it will always make you love it.