Hi Irri,
Yes you can build it there and take it on site afterwards, I use a router NATing off our main router to create a seperate network to build servers in the same way. From what you have described you will not be changing the IP when you move the server to the clients office so all will be fine. If you do end up changing the ip then do the following:
Install windows support tools from the CD
Open command prompt and type:
netdiag /fix
dcdiag /v
netdiag /v
repadmin /showrepl
Make sure DHCP is offering the correct DNS server (the DC)
