I was in the position a few seasons ago. I moved onto a lake with some avid skiers who set up portable courses frequently. When they found out I skied they quickly invited me to get out with them. Up to that point I had not given much consideration to my equipment as I was only free skiing trying to cut to get good spray. Silly me. After getting into the course a little bit I bought a ski that would perform at slower speeds in the course but would also ride well free skiing. I wish I had gone straight for the course ski. I also quickly realized that I would need a new (used) dd inboard boat, a comp vest, a masterline, and some gloves, hahaha. (No seriously I have bought all of those things since, except the masterline - that upgrade is coming very soon)
As for speed and length it may slightly depend on the size of the skier. I am 5'11 and about 210 lbs. I started skiing at 15off at 30mph in the course and only was able to make a couple of bouys. Then I slowed to 28 and started making better progress and was able to get 6 by the end of my first season in the course. This will be my third season and I hope to get through the course at least 15off at 30 mph.
I think any skier with decent ability free skiing will love the transition into the course. You actually have a way to measure how well you are skiing. Invite them out as a spotter for a couple of sessions before they get in the water and take some passes to break him/her in.