I use a Pentax 3.5mm XW for planetary and double star work, actually I've
nicknamed it my Mars ep. It's a pitty your budget wont allow for such a good
quality ep. It exibits zero field curvature

no chromatic errors

and the
lengendary flat field of the 5mm, 7mm and 10mm XW's

And the coatings
are superior to Naglers and Radians I've tried

I wouldn't trade it for any
thing else. I can place Mars half in and half out ot the FOV and it looks as
clean, clear and crisp as if it was on axis.
So if you can treat yourself, go for a short focal length XW, you will not
regret it. But a word of warning they are addictive
regards,CS