Pepperell, Groton. They border eachother, Groton will be more expensive. Anything east of 495 will sadly not be 'somewhat affordable'.
Both of these towns are quaint and beautiful, especially in the fall. quiet, peaceful, a reasonable amount of small restaurants. Convenient in that they border Nashua for bigger, tax free shopping.

If you can afford $2k in rent or more a month then check out:
Bedford
Lexington
Concord/carlisle
Littleton