Search around - plenty of people have 18s all around, especially for winter. There are a lot of wheel/tire combos that will fit.
https://www.crossfireforum.org/forum...ment-tool.html is a useful tool for figuring out the spacing vs. widths vs. tire sizes
https://www.crossfireforum.org/forum...b-removal.html will let you know how to take the **** off.
The cupholder is a legendary issue - either ignore the broken-ness, custom-fabricate something to go in that hole, or spend $50-$60 on the "storage bin" from a base coupe at a dealer and replace your cupholder with that. Instructions :
https://www.crossfireforum.org/forum...e-bin-how.html
As for your rear stabilizer mounts, a few other forum members have had that problem but it doesn't seem quite "common" yet. And Chryslers having that problem have nothing at all to do with it, because all of those parts are Mercedes through and through. At any rate, if you've got both sides re-welded now, you should be good to go for a while.
$500 is steep - it depends on what you're getting from a "tune up," though.
Oil and a filter will run $60-$100 depending on what sales you get - this engine does take a lot of expensive 0W-40 oil and the filter isn't the cheapest ($15-$25 for a real glass fleece one, which is what you should be using). Search for "oil change," "oil filter", and "0w-40" and you'll find plenty about how, along with debates about if cheaper 5W-40 oil is okay (and what oils meet different specs). I personally just stick with the 0W-40 - while it's a little pricier, the time and stress I save are worth it to me. It's easier to change the oil on this car than on any other car I've ever owned - the filter is incredibly easy to get to, the drain plug just involves taking off a simple cover, and you can fill the car back up through the oil filter hole, so you don't even need a funnel.
This engine does have 12 spark plugs and wires (2 per cylinder) and replacing the rearmost spark plugs is a pretty big pain, so that's another hundred and some-odd dollars there, plus if you've got someone else doing it they'll probably charge you for the skinned knuckles they end up with.
If you're getting new oil, new spark plugs, and new coolant or transmission or diff fluid, $500 doesn't seem incredibly steep - but if you're getting all that every tune up, you've got some serious overkill on your hands.