As far as additional models to buy:
Hoff: Warden(s) and an additional Watcher are really good investments, Ryle can also be good in some situations and you will want to get either a Mech Attendant or Mobile Toolkit depending on play style. Clockwork traps (once they are released) could prove to be worth-while as well.
Sonnia: Witchling Handler, Nino and/or Santiago Ortega, Executioner, Sue and an Austringer are all good investments.
As Hoffman tactica, starting out just tell him to do his absolute best at keeping all of his constructs as close to him as possible, use Machine Puppet on the Peacekeeper a lot and to not be over-aggressive as Hoff has a lot of defensive options and can often allow the enemy to come to him. Take a look at the Protection String mentioned in other posts on here also, it can make a huge difference. Learning and winning with Hoffman can be a little more daunting than the other Guild masters, but definitely worth it.