Ok so ill start with, i still dont know what questions to post where so if this is in the wrong place please move :)

Im looking at Wheels and Tires for my TE on Ebay and ive come across some "ALTSTADT RIMS deep dish 17x9"
with an offset of +20. I understand the less the offset the further out the wheels will sit from the car.. but will
these wheels be too wide for a magna? (illegal width etc)

I don't think 9" will fit, maybe with a +20 offset, but they'll stick way out. Also, you'll need probably 245's on a 9" rim, even then they'll be stretched. I wouldn't recommend it.

yes, they will be illegal unless you get some wide flares. You will not be able to flare the standard guards enough to keep them inside.

haha ok they are obviously a no go then :)
thanks guys.

Largest 3rd Gen OEM alloys, 17x7 ET46
An 8" wide rim with 235 wide tyres and an ET35 offset will be flush with the front guards. 8.5" ET35 ok with flares.

So anything 7-8" wide, ET35-ET45 should fit fine :).
Remember by law you can't go more than 1" wider than the widest optional OEM rim for the vehicle. Also can't increase the track by more than 1".

That should be enough info to get you going.

unfortunetly being a fwd the rear 'space' for wider rims is pretty much non existant. For ex i was running 20x8.5 in the rear but with a +40 or +42 offset..meaning very close to the body of the car. For that i didnt even need to roll the guards. With proper deep dish comes a low offset and the only way to fit them is cutting the guards..modding in flares and then repainting..looking at ballpark of $1500ish aka very pricey. So pretty much choice wise your looking at something in the higher offets and generic widths eg x8.5, x9 and offsets off lowest +30ish depending on width of rim