If you want to do the work yourself,look at Pella.com,and find out more. Pella does not look like a replacement from the outside,but with the frame thickness of their wood framed windows,you will need to have a transition moulding on the inside. Pella also has a fiberglass line.
The website is pretty cool,showing installation proceedures. Pella has installation available,but a skilled carpenter or DIY with patience may be able to do it. My suggestion is to do ONE window and see how much work it is,how the finished product looks,ect. As with any window,the installation aids are critical to your success. So is your own skill and understanding of installation quirks you are likely to encounter in a replacement situation,The thing I liked most about Pella,is that the exterior trim options allowed us to make the replacement windows look just like the ones in the new addition.
Not pushing one brand over another,just giving another option.