Wow Freddy, I don't know how that 400D compares to what you are using now (fully identical? yours is a 60 or 600, no?), but it sure seems like a lot of Euros for an Ha mod DSLR.
When I was doing more DSLR, I just bought a used/refurb D5300 in decent condition (think it came from Japan off eBay) and removed the stock filter myself to make it full spectrum. That was in the neighborhood of $400, plus a little more for a small electronics repair kit (think laptop repair, lots of tiny screwdriver bits and edge pry tools, etc.) from Amazon.
Mono, like from mono astrocams, isn't necessarily more time-consuming,
for equivalent results. The sensor response is usually greater, so often 20 minutes each of R, G, and B can be better SNR than 1 hour of OSC. But it certainly won't be worse.
The files should also be similar, based on resolution and bit depth, of a grayscale mono and undebayered OSC.
But while you will get more red response, especially where Ha resides, I don't know if it solves your underlying philosophical/theoretical problems as described and discussed in a number of threads.
I think it boils down to figuring out just what do you want to create/show, how, and why. I think we've said it before, maybe here or the Heart Nebula threads or both, but any combining of wide and narrowband has potential pitfalls, depending on how it's done. And Ivo I think explained it even better in the other current thread. Is there a good logical explanation for the NB+WB combination? For example a NB image but with RGB stars is a bit of a mash-up but you can reasonably explain what you have done, and both the NB and RGB portions of the combo remain legitimate in and of themselves.
Using Ha as the L for RGB is a mismatch, and so for any given target it may "work" (meaning you get a pretty picture you like) or it may flop. Blending the Ha into the R channel may be slightly more legitimate than that, especially if you can get an appropriate blend percentage, but is still an artistic augmentation of the red channel. The most appropriate, if one cares, is likely separate processing of BB and NB followed by a layer blending. Still very much an imager's choice result, perhaps still "artistic" depending on where you draw the line, but can be rationally explained to both yourself and the audience. The NB Accent module streamlines this exact thing for us.
If you want to keep with DSLR for now, you might look for a used, already Ha-modded, Canon of the exact same model as your mono-mod Canon, for ease of combining mono and OSC. I would consider getting an L or UV-IR cut filter to acquire true L with the mono for your BB. But I would probably really think about a filter wheel and full set of LRGBSHO (does not need to be expensive Chromas), since you already have a mono cam that you seem to like. Just get things in sizes that could be used with a future astro mono cam, if you ever do that.
Or, since you really like nebulas, just get SHO filters and do SHO plus RGB stars, the RGB stars being acquired with the stock DSLR.
From now on I will always wait two days to see if Mike replies 
) I will only give this on top: