So for example, if you have a 2.5-3.5 year gap, then you'd have DC1 in infant seat for ~12-18m, then into toddler seat from ~1-4y. When DC1 is 2.5/3/3.5, DC2 comes along and occupies infant seat from 0~18m. DC1 is at this point about 4-5 years old and growing out (if not already grown out) of the toddler seat anyway, so gets a booster seat, and DC2 goes into the toddler seat. Therefore you only ever need one infant seat and one toddler seat. When DC1 is about 6/7 you'll need a second booster seat for DC2, but high back booster seats are cheap, even for the good ones, so this is no issue.
But if infant and toddler seat are totally inextricable as in the Recaro system, you'll definitely need a second infant and/or toddler seat. And as you've found, both of these are expensive! You can get cheaper infant seats, but in that case, why not just get a cheap infant seat + separate rotating toddler seat for DC1 in the first place? Unless there is something else specific about the Recaro you like, I'm not seeing the utility in it. There are loads of rotating seats on the market now and will be even more/cheaper/better in 18 months' time (as a few being released this year).
Secondly a note on sizing: If you think your child is likely to be long and lean, the Recaro seat (toddler/rotating section) has a standing height limit of 105cm, which is roughly around age 3.5 for a tall (but not giant) child. Other 0-4 rotating seats which go by weight rather than height will last you a little bit longer (months, not years). You might find yourself looking at seats in the larger size which are either rear facing (e.g. Britax Max-Way) or forward facing (e.g. Joie Bold) but the harness goes up to 25kg, which by height is 125cm, for a tall child maximum around 6-7 years of age. (This might be one reason to consider the Recaro system despite not being able to pass individual parts down to younger sibling).
In addition, the infant carrier portion of that seat is only suitable up to 76cm. This may be a problem if you have a long baby - because they will likely reach this height by around 9-10 months, possibly earlier, and have to transfer into the main part of the seat. (It's also quite difficult to measure height on a baby who can't yet stand, but that's by the by.) Not a problem for safety as they can still be rear facing, but can be a problem for comfort, as they still tend to nap in the car at that age, and a lot of the toddler seats are much more upright and don't recline as much as the infant seats, unless they are suitable from birth.
I would be cautious about this idea that you can cover all bases from the start. Manufacturers like this very much because of course it guarantees that you as the parent will spend a huge chunk of money (on that second stage seat) with them as a company, rather than getting to the end of the infant stage and having the whole market to choose from as you would otherwise.
My advice would be in general, either get a 0-4 seat which stays in the car, but don't pay extra for a clip out part for the infant carrier (as this is a false economy, IMO), or get a standalone infant carrier (compatible with most pushchairs on identical adapters: Maxi cosi, Cybex, Joie, Nuna, Besafe, Recaro), with a base if you want one, and then research and decide on the toddler seat (0-4 or 1-6) as a separate decision closer to the time. For most people I think the latter is the better choice, but TBH some infant carrier + base combinations can cost as much as the rotating 0-4 years seats by themselves and in that case if you like the rotating feature and don't need the removable infant seat, why not.
There is nothing wrong with the Recaro seat, it's a very clever design, and the safety rating on it is good, I just think it's expensive for what you get, and from what I've seen from reviews etc people seem to find it cumbersome and impractical in day to day use. However, some absolutely love it and rave about it. If it's definitely the right combination for you, I wouldn't warn you away from it. I would just say be aware of the actual projected age range (for each part of the seat), the potential to pass a set of separate seats down, and the height of it.