I am looking at a 312 with 454's at 310 HP. The boat does not have the Latham steering. Will this be a problem on this boat and what should be expected without it. This will be my first high performance boat and have been told this one will not go fast enough to need the external steering. Do these boats come with any standard power steering or can it be added?

Yes, there is standard power steering. Usually the standard merc 454's have 330 HP each assuming a performance application...

Regardless, the boat should be in the mid 60's at the highest. External steering is nice, but not necessary at those speeds.

As long as you plan to keep the motors stock, an external tie-bar on the drives (which it should already have) is all that is needed.

What year? (not that it really matters though). I had a 312 with 502's and all the options. This one is a 310HP version of the 454 and not a good candidate to do any motor work (the older carbed 454's were 330HP). It will be a low 60 mph boat but will ride quite well. Hydraulic steering isn't needed on this boat but is always a nice item on any boat.
IMO:
It seems this was a base model 312 without options. If it's a 1998-2000 you should be paying in the 40's...a little newer might get it into the 50's.