We had a combo unit, and one did experience an early demise. We were a little annoyed with the extra expense of buying another combo unit however what we discovered is that would have been a deal compared to what actually happened. Nothing on the market at that time would fit the cabinet we had, so we needed to replace that tall cabinet. Matching existing cabinetry is difficult, and we were getting no help there. In the end, the dead appliance morphed into a complete kitchen remodel. This time, we're doing our ovens as separate units, and we're making sure that the oven stack is made with separate components, so that they could be unassembled and remade in another size. What we had before was one tall cabinet, floor to ceiling, that was impossible to alter without destroying it because of the way it was constructed.

So, make a niche(s) with the proper electric service and size to have a built-in put in if you really need it to sell (but I bet you won't)--if you're planning to sell within three years, get the built-ins already, but otherwise, they're going to be just so many used appliances. If yours are truly ratty, you can buy new versions of each. If you WANT a speed oven, and intend to use it as such, that's another story. Go for it! Read through the manuals, etc. But I think you'll find that it's not at all the same thing as your Breville toaster oven. You can use the convection oven settings on a speed oven to perform some of the same functions, but it's a real oven, with limitations, not a toaster that tap dances. I sometimes use my speed oven the way others use a toaster oven--mainly for heating up frozen appetizers or reheating casseroles when other ovens are full, but my old (dead) toaster oven was beloved for making great toast, which a speed oven isn't, and not for other uses. I do zap sandwiches on MW mode, and once in awhile lightly defrost or melt, but if I used a microwave more, I wouldn't have such an expensive machine doing it. I agree with putting enough thought into a remodel that it won't limit resale, because you never know what the future will bring--thus make sure there's place for built-ins to be built into--but unless it's a flip, it should be designed for your own use and enjoyment, and you used the key words "heavily use" and "love".

We have the full size Breville toaster oven bought for the kitchen reno and the Bosch speed oven. I really couldn't live without either now. I guess you could toast bread on broil in the speed oven but that mean you would have to get the rack out and it's just not as convenient or time saving. Having the versatility of both ovens makes me happy. We also have the side opening Bosch oven which is the star of the kitchen regarding how awesome the food comes out of it and ease of use.

It appears that the Frigidaire FGMO226NUF is to be built in only when installed with a trim kit which increases its width to at least 27". The KitchenAid KMBS104ESS and Miele M6040SC (among others?) do appear to be fully installable in a 24" cabinet. Installing a 24" speed oven may be the way to go. You must carefully review the documentation to make sure that the oven can be installed under a counter, under a cooktop, and does not require a 27" or 30" trim kit for installation. Note that the Bosch HMC54151UC cannot be installed under a cooktop. The Miele H6200BMOBSW or Fisher Paykel OM24NDB1 (among others?) might work.