Without suggesting anything is afoot....I'd question one advice about such an expensive cure to a problem of growing grass.

At least, get more than one estimate of a job that may be questionable in your mind.
Many fixes are well founded....it can be often said that a built-in irrigation system will help the homeowner avoid mistakes of watering.
But....if the homeowner isn't adverse to picking up a hose and turning on a tap, such system can be very expensive.
If the area suffers from freezing weather, then irrigation systems have to be maintained in an orderly fashion. Water in the pipes has to be drained sot hat winter freeze wont harm it.

Again, irrigation systems, when it works good, is without question, a great way to maintain a lawn.

RoundUp will kill everything greeen....grass, weeds, shrubs, ....everything. But, where it is called for, Roundup can be the beginning to a healthy lawn.
Herbicides will not control lawn pests...although it can kill beneficial bacteria and soil insects that we wish to keep.
Worms, for example, will be harmed by a herbicide...so the advice is to keep such use to a minimum.

What might be important is to use a seed that is the best you can afford. Good seed will grow in the poorest of soil.
Bad seed, wont do well in the best of soil.

I'm not familiar with "wiregrass' so I wont comment on how best to treat it.
It makes good sense to dig up a yard before you plan on growing grass....of course....like putting in a road before you add the sidewalk....but that is also maybe just a selling point for doing the job.
Do you need....or want ...such irrigation system.
If not, then maybe investing in a good rubber hose will suit.

Grubs are with us all the time....its why sometimes we see skunks, squrrels, birds, rabbits....on our lawns...they're looking and scratching up a lawn to dine on the grubs.
There are methods--chemicals..which can be used to kill grubs. But you should understand how these pests invade our lawns and when the best time to do it.

Thanks for your response. I will follow your advise with respect to the system, etc.