possibly outside of line-of-sight.
You’ll not get this (at all reliably) on your budget. You’d really need something higher power (which requires licensing to use, fairly straightforward for lower power bands in the UK, not sure about Europe).
In the same house over three floors, it’ll depend on the construction of the house but they’d probably do the job. Should be a fairly quiet environment so that’s not so much an issue.
longer range is no always better, you get more traffic from other users.
Most kit that does longer range also lets you code the channels, not had this be an issue other than on the cheap sets (cos they’re pretty common for leisure use).
Though rather than spending money, would mobiles not do the job? And if you have wifi, skype/google hangouts/facetime audio calls for free?