The Rotax 912ULS and 912IS of similar vintages are pretty similar in climb and cruise preformance. The IS is more fuel efficient in cruise.
Is the 914 still an option ? Look at the 915 for a fair bit more in cost.
The power requirement/backup issues of the IS seem well known and allowed for and the 912IS has been flying for 12 years.
The both versions of the 912 seem ideally suited for either 600 or 760kg.
Are you really looking at high attitudes ? A lot of these aircraft have limits of 13,000ft (ish)
The Sling 2 is both an LSA at 600kg and a VLA at 700kg and was flown around the world in 2009 at 900kg each day for 40 days. It used a 912ULS.
With this aircraft going to class G this will now (soon?) allow you to take 2 people with full tanks.
If you want to fly both engines then Fly Illawarra have an Evektor Sportstar with a 912ULS and a Evektor Harmony with a 912IS.