Great decision by buying the gravely. I do lawn care for a living, and ill assure you that it is the greatest mower on the face of this earth for all around price/value, sturdiness, and cut quality.
Bought mine before spring in 2011, put 480 hours on it so far, no problems at all. Never used the choke, even in cold weather.
I mean I mow for a living and it has held up for me just fine, so that should assure you, as a homeowner, that it WILL stand the test of time. Just take care of it.
As far as your filter question, be safe, use the OEM filter. when you clean the filter out, DO NOT spray it with a air hose. tap it a little on the side of something clean and hard surfaced, until theres no dirt.
My set up:
-2007 Chevy 2500hd 8.1l 4x4
-7x16 open trailer
-2011 gravely zt 52hd
-21" craftsman push mower
-Stihl trimmers, blower, edgers, chainsaws.