There no such thing like solaris key. You register at sun.com and pay for subscription to receive latest security patches and other patches for Solaris operating system. You get some id at sun.com and then you can download those patches. Without this you have unsecure and unstable version which might work properly but doesn't have to.
First install Sol10 on the machine and then check whether it works properly (doesn't hangs and etc). If it does, it ok.
Docs for machine you mentiod above :
http://dlc.sun.com/pdf/806-5980-11/806-5980-11.pdf
You connect with HyperTerm from Windows (use RollOverCable) to Serial port B.
See page 17 and 23.
Then connect Serial A to ethernet switch and config this port from session to Serial A to either receive address by DHCP or set it statically.
THen you can connect to ALOM via SerialA port by using ssh or telnet or http browser by IP.
Then from ALOM you have to access OpenBoot PROM (ok prompt)
see page 34, how to get there and set env variables to boot server.
OpenBootPROM is like BIOS in x86 platform.
If you have up and running Solaris on that machine and access to ssh or telnet session the best way to reinstall would be liveUpgrade. There is plenty info about liveUpgrade mechanism on the bigadmin.com
Unless you need clean installation of Sol on that machine you need then use JumpStart mechanism. Also docs on BigAdmin.