Get as many replacment quotes as you feel comfortable.
As a consumer, you do not have to purchse on the spot.
Understand the sales tactics of the sales person. If the sales person tells you "the deal is only good until Friday" or they continue dropping their prices on the spot, they are looking to sell you on the spot. BE CAUTIOUS.
Make the sales person talk about the windows they sell - not their competitors windows. Sales people that sell negatively against competition are avoiding talking about the pros and cons of their own product.
Be mindful that a sales team will want to sell whatever windows they offer, even if they aren't the best fit for your home.
It is always recommended to have your windows installed by whomever you chose to contract with (not subcontracted). This protects you - the buyer - from any warranty issues.