Hi Andy,
with f/5 you own a fast optical system that requires well corrected eyepieces. If I were you, I first would buy one eyepiece of good quality and complete the ep collection successively. My recommendation:
-- 5-8mm Speers Waler Zoom (new series coming soon) with >80° FOV for high magnification
-- 13mm Hyperion with 68° FOV in the mid mag range (gives you about 2mm exit pupil, very good for most deep sky observations)
-- 22mm LVW (65° FOV) for low mag, one of the best corrected eyepieces for fast instruments (in my opinion)
-- A low power ep for finding objects, the most expensive part for owners of fast optical systems, in the 27mm to 35mm range (depending on night sky quality).
Just my 2 cents
