Advice / suggestions Initial setup new server

Please some advice and suggestions for my server initial setup.

Situation :
- myowndomain.com ( b-smarthosting.net )
- Static IP
- Modem
- AIRPORT extreme ( static IP and DHCP active for clients ).
- Backup medium ( USB connection with AIRPORT extreme )
- MAC mini server ( new ) needed for SBS purposes ( ethernet connection with AIRPORT ).
- Client MAC & Windows ( 5-10 connected with the AIRPORT for internet )
- Printer ( all in one) .
- 20- 40 mail accounts

server purposes :
file sharing, web service , e-mail service etc.
Must all reachable out site the lan ( mail, files ).

Future :
Possible second domain.

My domain registry is running the DNS server .
( ns1.b.smarthosting.net and ns1.smarthosting.net ).
( DirectAdmin for control DNS )

I do not have a second server name server in use ( one server one location in use ).
I wish to run the e-mail service on my own server ( and Sendlabs MX backup Dyndns for backup ).

How to setup ?

DHCP : manuel ( airport is DHCP server connected to modem )
IP adres : 10.0.1.5
Router : 10.0.1.1
subnetmask : 127.0.1.1
Search domain : ( where is this setting for ? )

Next screen : enabled airport extreme auto management

Next screen :
Primary dns name : ns1.b.smarthosting.net ( my big question ; is this correct ? )
Computer name : osxserver

records DNS smarthosting.net :

mydomain.com NS ns1.b.smarthosting.net
mydomain.com NS ns1.b.smarthosting.net
mydomain.com A mystaticipadres
www CNAME mydomain.com
mail.mydomain.com MX 10

( later also the Dyndns record for mailbackup )

PTR record needed ?

Or is it better to chose osxserver.mydomain.com as primary DNS name ( do not have second name server ).
And change the NS record in DirectAdmin to my own server ( ns1.b.smarthosting.net to mydomain.com )?
And make de records ( www, subdomain.mydomain.com ) in my own DNS server.

Thanks for responses / suggestions