To reach maximum heat output, the baseboard needs to be fairly hot, but during most days, it could take the chill off at lower temperatures, especially when the house is well insulated and has no drafts. The in-floor radiant bits would likely need different temps, too, so you may need three different ones. The in-slab stuff would likely use the lowest, followed by the floor radiant, then the baseboard. Each zone, individually, is likely less than the lowest modulated output of the smallest boiler. I would not consider a combi, as you'd be keeping the thing hot all year, not a great idea. I don't have any personal experience with the Ultra, but when I was looking several years ago, the installer said he would not install one, since he'd had too many problems with them. This is one person's opinion, and I cannot say if they've made any changes, or it was valid in the first place. I did end up with a Buderus unit, and have been quite happy with it.
On most boilers, where the indirect is set up as a priority zone, it alters the boiler output temp for the task. You can also achieve different temperatures with a mixing valve, as long as the output is at least as high as the highest value needed. These modular boilers have all sorts of options, controlled by plug-in boards to do nearly anything you might want. It can get expensive, but it's hopefully a one time expense. One Viessman unit I looked at had an external mixing tank that could have the temperature automatically set to whatever was needed. So, depending on how complicated you want to get, you can exactly match your needs.
A condensing boiler's return water temp is a big part of how efficient it is, if you can match the outlet needs, and the return water's temp is cool enough, it can reach its max efficiency.
Someone with a lot more experience than I should be able to help with some specifics.