I have an 2007 GT and I'm very happy with it. I have put 81000 KM on it, not sure what that is in miles and haven't had any major problems. Just a few minor things that warranty took care of. I just read yesterday that Ford just took the spot of number one in car sales in Canada. I know they had a period in the 80s and 90s when the cars were pretty much junk IMHO, but they are doing things right again as far as I can tell.
As for easy to mod, there is lots of aftermarket stuff out there for them. I heard a statistic that out of all the sports cars sold in America, 50% are Mustangs so there is a huge aftermarket. When I look under the hood of my car everything has a very simple layout to it and it looks very easy to work on. I say looks, because I haven't had to work on it of course.
Personally I really don't like the 2010 Mustang style. I think they nailed it with the 2005-2009 but I guess that is a matter of opinion. I think they look bolder and more aggressive, I love the big tail lights and big headlights. Anyway, that is a matter of opinion so go with what you like.
Since Mustangs are so common, they are not all that expensive to buy used. Here in Canada you could get a used GT like mine for about $20000. I'm sure it would be less in USD.