30 seconds is about the minimum i reckon. Push up is related to drop time, chicksands was under 5 min for a 30 sec course, the wentwood one was about 1m-1m30s course with 15-20 min walk. A 30 sec course with 20 minute push wouldn't be great!
Proper timing is essential for short course stuff with lots of people as the gaps between people will be very small. However i do a few races a year at a local forest where timing is 2 stop watches and some radios, not the most accurate but with 30-40 entries it doesn't matter so much.
First aid required as well but you don't need to pay for extreme medics of whoever just people with qualifications.
If you are doing a series then you need to make some minor changes to the course to keep people interested. just sections rather than a whole course though.
Entry fees wise the very local one with manual timing is £10 with a bbq. Chicksands was £20 each i think. £25 would be my max and for that it'd need to be a good venue. Talk to Naked racing (Chris Roberts) about timing costs etc as he does it for various people. If you need a number drop me an email.