Honestly just get yourself some lowering springs and some nice wheels and you got yourself a nice, clean car. Maybe get artistic and paint your calipers /personalize it a bit. Other than that you don't really need to go crazy, they're awesome looking cars even when they're stock.
Have fun with it
But if you do want to go crazy, like many of us here there's a decent amount of options!
Edit: didn't know they censor the word rice-er haha
if you want street cred. just get some 19x9 wheels. lower it all the way. take the front bumper off and add a bunch of stickers (crooked on the back windshield).
The one guy who i saw on the forum here on, coilovers w/ 19" wheels' had one of the best looking cars at the time. ^and although im making fun of a certain style, done right-- it does look good.
here's some functional stance.
If your broke like most of us though, get lowering springs and a decent set of 18's.
I think lowering the car - whether by new springs or coilovers - helps the look of S40s more than anything else. The ride height of a stock S40 is so high, just astronomically, with huge fender gaps. And a set of springs isn't that expensive either.
^that made me giggle. But it is a good, simple look on these cars
And they seem to be around $250-400 for new ones. But with some searching you may be able to find some second-hand ones that are practically new (or still in the box) for a deal. Elevate, Eibach, H&R are the brands I'm thinking of at the moment. I have Elevate on my S40
I'm still looking for one in the U.S. but I think the badgeless eggcrate grills look awesome. Fogs if you don't already have it and the silver turn signal bulbs for the front lights look good. There are lots of little accents that look great just browse people's threads