I use my boil kettle as my mash tun for single-vessel BIAB. It works fine. Somethings to consider are:
a) You can get grain bags cheap on Amazon, but don't go too cheap because nothing gets your attention like 20 pounds of wet grain splashing back down into 150F
water sticky, sugary wort because the drawstring on your bag broke.
b) If you're doing 5+ gallon batches, you will want a hoist or some other assistance in lifting the grain bag and holding as it drains. I have a 150-lb-rated ratcheting pully that I use for big batches. Other people use a strainer the same diameter as the kettle. Whatever works for your setup. If you're doing small batches (my standard is 4L), you can skip this requirement.
c) While not absolutely necessary, waterproof insulated gloves make handling a heavy bag of piping hot, wet grains much easier. And you can give them a good squeeze to get more of the wort out and reduce your losses to grain absorption.
d) Batches above 5 gallons have enough mass not to lose too much temperature over the course of the mash if your ambient temperature is fairly mild, but you may want to insulate your kettle with something non-flammable if you're worried about holding a precisely consistent mash temp. Small batches need insulation more because higher surface-area-to-volume ratio, small mass, etc.
e) If you're going this route, crush your grain finer than you would if you were going to sparge/lauter. It will help with your mash efficiency.