The problem with HERMS is that the mash temperature will always be lower than the HLT temperature due to heat loss. I have a RIMS system, but I have designed and ran a HERMS system for a local brewery.
The HERMS was slower to raise temps than I was expecting, but it could maintain pretty well. The best thing to do (and I do this on my RIMS) is to heat strike water so the grain bed is slightly lower than the intended temperature and then bring it up with the HERMS coil. The temperature difference between the HLT and mash determines how fast the grain bed temperature will rise, so a mash of 150F will need a HLT of 152 to 155F. The smaller the difference, the slower the response time. But with a smaller temperature difference, the control will be tight. It's basically a balancing act. In my RIMS system I don't worry very much about striking at a lower temperature, I worry more about getting too hot and denaturing the enzymes.
So If it were my system, I would strike a few degrees too low and slowly raise it with the HERMS coil. The temperature of the HLT will depend on heat loss in the mash tun, how fast you want to raise the temperature and how tight you want to control the mash temperature. I personally like the tight control and would rather wait to reach my desired mash temperature and not overshoot my mash.
Step mashing may be a bit harder to do with a HERMS, but it can be done. It's going to take a while to learn how to brew on it and dial it in, eventually you will hit your stride.
Patience! That's the part I hate, I want it now.
