Do you have pets or children to worry about investigating baits?
As far as stopping them getting in -- make sure your window screens are snug, check on your weatherstripping (helps with heating/cooling costs, too), try and find every ingress-egress point you can and plug it up.
In times of insect invasion - diatomaceous earth is my frontline. Obscenely cheap inert dust of pulverized diatoms, it recreates "that scene from Die Hard with the broken glass" but for bugs, which then dehydrate and die; as with any dust, don't breathe it or get it in your eyes, but it's nontoxic. Apply near doorways and in corners (under the oven, the corners in the pantry, behind the toilet). This is my go-to for roaches especially if you pair it, on a smooth floor, around the next item:
If you need baits, you can get a $2 bottle of boric acid dust at the hardware shop, mix it with peanut butter or sugarwater in a bottlecap; ants & other foraging insects will go for it and subsequently perish.
Flies of most sorts will gladly drown themselves for you if you mix a bit of water, a bit of wine, and a drop of dish soap in a cup and leave it uncovered for them to investigate. Keep these traps indoors, you don't want to drown bees.
I also recommend letting your spiders and predatory bugs like assassin bugs, lacewings, dragonflies, mantises, even wasps, live in peace if they aren't interfering in your life. I typically have a couple spiders in the doorways and windows taking out the few creatures that do get in, and essentially never see any bugs in my home NOT already-deceased and directly below their webs for easy vacuuming - and I keep a compost bin full of rotting plant matter all of four feet outside my door since all I have is a balcony. Amphibians, bats, and insectivorous birds are good company to encourage - so don't eliminate all their cover (a few patches of higher grass, thicker bushes, anything that would create a humid toadhole) especially if it's a little away from the house, avoid spraying insecticides (especially if not contained to the areas you need them), build a bathouse, tolerate small to midsize wasp nests that aren't right over your door. See if a few "Lucas the Spider" videos get the fiancée to decide they're her friends.
Prevention is far less work than fighting off an infestation, but it also doesn't have to be expensive or high-toxicity, and is easily DIY.