wheels offset

Hi guys!
I'm going to buy a set of Forgestar F14 Front 19x9 Rear 19x10 Offset +35 front and back but I'm not sure if the offset is proper for an e46 M3.Could you help me?Thank you!

Stock offsets for the E46 M3 are 47 front and 25 rear. With the increased width, I suspect you will be OK in the front but I think you will need a spacer and longer bolts in the rear.

It's been recommended to me to stay away from spacers and I quickly agreed by my own assessment. It's only going to make the connection of wheel-to-car weaker. Longer bolts are surely weaker when they pass trough the spacer with just their thread and the spacer introduces another shear-held interface (and one subject to greater leverage, no less).
I would want to get appropriate wheels from the start that do not require modification to fit... this is a very important connection, after all.

You will not need any spacers with an offset of +35. Everything will fit but the position of the tyre in relation to the fender will not be the same front to back. The front will be pushed outwards towards the fender giving you a wider front stance (will reduce understeer). The rear will be 10 mm further inwards (away from the fender). A spacer can be used to make it look better but 10 mm is virtually not noticable.
I use a square set up for my track car with a +35 offset.
As long as you have spacers smaller than 15 mm you will have no issues regaring weaking wheel to car issues, but you are right getting a set of rims which do not require spacers is always the first choice.