I need to establish an absolutely reliable line of verbal communication
....
I have tried a walkie-talkie T5950 by Motorola. It is the most powerful I

From what I can find on the web, this radio only puts out 1W, which is not
much. Handheld radios can legally go up to 7W, though very very few go above
5W. If your communications are possible but poor at 1W, they would probably
be acceptable but maybe not crystal clear at 5W (you will not get 5X better
signal because when you double your power, your signal only improves 1.5X).
The problem with consumer-grade radios is they seldom tell you the power
output on the box or bubble pack, so you may have to do some research to
find radios that do 5W. When they say "6 miles" or whatever on the package,
that is completely meaningless.

Be aware that only the 1/2W (500mW) FRS radios are license-free. Your
current radio and any other radio like it that puts out more than 1/2W are
GMRS radios, and require you to obtain an $80 license from the FCC in order
to legally operate them. Since the FCC will not issue a license to a
business, one person at each of your locations will need a license (unless
you are related; family memebrs can share GMRS licenses).

If you don't want to sink $160 into GMRS licensing (the licenses are good
for 5 years), you can look at license-free MURS radios. These radios only go
up to 2W though, so I'm guessing they will not have the power you need.
Also, the radios themselves cost $100-$200 each.

Most CBs only put out 4W and CB has other issues too (interference from
CBers three states away). If you can find a couple of inexpensive CB radios
that do SSB (not all do, read the specs carefully!), you get 16W of power in
SSB mode. That would probably do the trick though you might find yourself
changing channels once in a while to get away from interference.

What power output do I need? What kind of antenna?

I think the simplest solution is to find a pair of 5W GMRS or 2W MURS
handhelds that have detachable antennas. If these still can't make the
distance, you can purchase aftermarket antennas. These will be long and
ungainly, but will about double your performance over the stock stubby
antenna. When you look at aftermarket antennas, try to get one that
advertises a 5/8 wavelength element, as 5/8 has more gain (=better signal)
than the more common 1/4 wavelength element. Aftermarket antennas can be
found at a commercial radio shop (the place where police depts buy their
radios) or at a ham radio shop. Make sure you get antennas tuned to
462-467MHz for GMRS or 154MHz for MURS, or you will fry your radios the
first time you transmit.

Hope that helps!