Yes I think you can strap the Cloud Z in with the seatbelt if you want to do that - the base is so expensive!
As for the next stage seat, you could get a second base and put the Sirona Z on it, or for less money you could get a rotating seat with the base built in. Most of these are actually suitable from birth so you could keep the Cloud for your main seat and have the birth-4 years (ish/18kg/105cm) seat full time in the second car until the baby outgrows the infant seat and then look at swapping it or getting a second one, especially if you think you'll have another DC.
Cheaper than seat + base (£400):
Cybex Sirona S i-size with sensorsafe
Maxi Cosi Mica
Joie i-Spin 360
Joie i-Spin safe (RWF only)
Recaro Zero 1 elite i-size (includes pop out infant carrier)
Recaro Zero 1 i-size (without infant carrier, suitable from birth)
Britax Dualfix i-size
GB Vaya 2 i-size
GB Vaya i-size with sensorsafe
Nuna Rebl Plus
Nuna Norr (RWF only)
Silver Cross Motion
Cheaper than seat alone (£260):
Joie 360 Spin
Joie Spin Safe (RWF only)
Cybex Sirona S i-size (nb, this is the same seat!!)
Britax Dualfix 2R
Or you could even (later) get a toddler seat which does from about 1 year all the way up to 6/7 years rear facing (25kg) for the same money:
Under £260:
Britax Two-Way Elite
Britax Max Way
Britax Multi-Tech 3
Axkid Move
Diono Radian 5
Under £400:
Britax Max Way Plus (sometimes dips down to £220ish)
Axkid Minikid
Besafe izi Plus
Klippan Century
And for another option (in case this isn't enough :o) there are infant carriers which are cheaper than the base (£140) which you could keep in the second car at all times:
All Joie infant seats except i-Level (For Gemm/Juva the clickfit base also comes in under budget)
Britax Babysafe
Maxi Cosi Citi
Maxi Cosi Cabriofix
Maxi Cosi Tinca i-size
Cybex Aton
Cybex Aton M i-size (fits longer)
Silver Cross Simplicity
Hauck Comfort Fix
Graco Snugride
Graco Snugride i-Size
0-4 years seats which are cheaper than £140:
Joie Steadi
Joie Tilt
Joie Stages
Britax First Class Plus (but only RF to 13kg)