I was thinking about getting a used Mazda 6, since the rx8 is a bit expensive. Is it a good car to modify? My other option was Accord. But I wonder before I buy a car I might as well get some comments about the Mazda 6 or even Mazda 3  

I really dont know much about these cars, but I really like the way they look a lot more then an Acord. Plus I have seen one with an aftermarket exhaust, but I would like to see one with some other mods.  

Mazda 6's aren't really tuner cars. You won't find much aftermarket support for the vehicle and you'll probably end up getting sick of having to custom fabricate everything yourself or have local shops fab you stuff. You'd be better off getting an old RX-7 if you want a rotary tuner.  

my wife drives a 6, if that gives you an idea of an answer to your question lol.

honestly the 6 is a great car for a daily.  its got a solid platform, double wishbones front and rear, reasonably stiff chassis.  they do very well in the speed challenge series actually.  but as far as buying one to modify, there's definitely better cars (but the accord is NOT one of them).  

it sounds like you're thinking of 4 door cars in the $10 to say $14K range.  think about a WRX.  

i would go with the accord it has a stong motor that can stand the abuse of after market parts. plus you have alot of after market support. and depending on the type of accord you can get in a v6 vtec or i4 vtec both are really good for tuning.you dont really see too much mazda 6s and i doubt it will last as long as the accord and they are not as fast stock. but really what matters is what you want