Yeah contact whmcs and tell them what is your needs

Host Bill App can do your request really fast if you ask them on the forums.

I would choose your control panel first to make sure it's compatible and works well together. If you use cpanel then check out lpanel.

nothing will be perfect - it like looking for the perfect wife (OR husband) - its never really perfect - deal with what you have - do a few manual things and request the changes to WHMCS

Pay for XX months and get XX months free

for this you can make promo coupon 100%off for monthly payment for any service

Did you look at HostBill ?

I used modern bill before but hd a lof of problems.. I use Ubersmith now and no problems in 2 years..