Somthing else that might be of some help.Ultra Running magazine has lots of race reports and results, where they give a 1-5 score for trail conditions and 1-5 for vertical gain.
Most sources are missing so many races
Where is the Chilly Chippewa 50k, John Dick Crusty 50k, Keyes Peak 50k, T-Bunk Endurance CHalleneg (50k, 50M, 100M, 150M, 200M), the 6 hour race in Green Bay, The 8 hour trail race in Hartford WI, the 6 hour race at Goose Island (La Crosse), the Glacial Trail 50k/50M, The Frozen otter 100k
So the above source has 6 races and I count at least 9 races missing ... maybe more?
Dances With Dirt Devill's Lake 50k/50M
The big 4-0 Eau Claire
Ice Box 480 (8 hour) River Falls)
So I count 19 total ultras in WI and
This source picked up 13 of them - with the terrain / elevation (1-5) so better than the 1st one I looked at
2013 Goals ~ Mar < 3:00, 5M < 29, 10k < 35
Another vote for Dirty German 50k. It was my first ultra this past May and it was a great race. Good first trail 50k, as the park is not too technical or hilly, so it will ease you in.
Another source you should consider:
They are based in Delaware and put on some amazing races. The Delaware Triple Crown and Trail Marathon (while not ultras) are awesome races to consider for the spring.
My Blog: trailsandcocktails.com
I Run for Oiselle
3/2/14: NERRC Winter 10k
3/30/14: Love Run 13.1
4/5/14: Tyler Trail 10k
4/13/14: Healthy Trails 5k
5/4/14: Broad Street Run 10M
5/9/14: Ragnar Relay Cape Cod
5/18/14: Dirty German 25k