Hi Paul,
I only use old primes and love 'em!
I see you have a 50mm so you should look at maybe a 24mm or 28mm and possibly something around 85mm to give you a good range of focal lengths.
The old Nikon lenses are fantastic and you should be able to pickup a decent 28mm for that money
http://www.kenrockwell.com/nikon/28f28ais.htm the f3.5 is good but the F2.8 is a diamond! I would also suggest looking at some of the less popular brands (but do some homework before buying). I have a cheap Chinon 28mm F2.8 that works great for video and stills. Same as this -
http://www.ebay.co.uk/itm/AUTO-CHINON-2-8-28mm-lens-M42-for-Pentax-Canon-Nikon-Olympus-/251258695184?nma=true&si=NGmWOBer6DPiMUhV%252Fs1yN0%252FjLtI%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557With older lenses you wont get any stabilization so anything longer than 50mm will need a tripod or shoulder rig for video use.
I would also suggest looking for a faster 50mm. There are plenty out there. On a budget I would recommend a Revuenon 50mm f1.4 which go for around $80-$100 on eBay. The extra stop makes a huge difference in low light and they're built like a tank. Go for Pentax-k mount and get an adapter. From the lenses I've owned, the K-mount versions always seem a little better (sharper, less CA etc).
Keep an eye on eBay because occasionally people sell off old Leica and Zeiss lenses dirt cheap.
Avoid the older Canon FD lenses unless you want to pay to have it adapted properly
http://www.fdtoef.com, with FD-EOS adapters you lose about 1 stop and get an increase in focal length.
These are good sites for info and reviews:
http://www.mflenses.com /
http://m42lens.com /