Engraving is one of the easiest things to do with a CNC. The cutting forces are almost nonexistent, so even the poorest of machines can do it.
High spindle speed is good because it makes the job go faster. That said, you can engrave on machines with 4000, 2000 or 1000 RPM spindles. It just takes that much longer.
The 770 or 1100 are both more than enough machine to engrave. If you start doing a lot of engraving, you may want to get a second high speed spindle to decrease your cycle time. But for the occasional engraving, you'll be just fine with the 770's regular spindle.