I am a painting contractor and was a juror in a lawsuit where the installation of the windows was at issue- Some of them leaked because they had not been installed and caulked properly. They had to be reinstalled. Because it was done by a contractor, we voted to reimburse the plaintiff for replacement and repair of the water damage.
So, I you install yourself, make certain that you do it exactly according to the manufacturer's specifications. Specifications, especially in construction, are there for a reason.
Also. you have to remove and dispose of your present windows. I live in California where labor is not cheap but decided to hire the window company's staff to replace some windows. I think that the labor per window was about $250.00 - for removal and disposal of the old windows, prepping the frames for the new windows, installation, putting the window frames back on and caulking.
Because it was only two windows, the cost per window was more than if I had installed more, but for the two workers it was a full day. I was too far away for them to get to another job.
The Yarders who said all that was easy to do properly- I don't know.
Have you ever done it before? Do you have a single- or two-story house?
The caulking is really important. Use a paintable silicon caulk and do not draw it across the crack.
Put the tip of the caulking gun over the space and pump the caulk into the crack until you can see it coming out ahead of the nozzle. Keep pushing forward that way, always having a little wave of caulk visible ahead of the nozzle.
Do not push hard on the caulk back into the space. Let it extend a little outside the crack.
If you do not do all that, the entire inner space will not be filled. The caulk will contract and harden and result in water damage that can rot out the sills from the inside.
Paintable silicon caulk is a little hard to find. It is at Home Depot in California.
Also, prime and paint all the surfaces of the window frames. This will prevent rot if water gets past the caulk.