Interesting project. Have you done a search on "casting glass" yet on a search engine like Google? Getting a oven large enough to cast the glass is the first problem. Then you need a controller that can drop down through the annealing temp slow enough to allow the glass to properly anneal. I started a search and found an oven for $840 but it`s only about 21" in diameter so that isn`t big enough for you.
I hope that you aren`t including the tools needed in the price of the telescope project!
You can do plate type glass (lots of broken glass is needed as you probably want a 3" thick mirror or deeper if you do a ribbed one like the Hale telescope mirror).
I would suggest that you refrain from fusing glass pieces as Richar Schwartz (sp?) has been that way for a while and he`s already learned that plate glass doesn`t work for that and you need pyrex in order to not have it shatter at the least temp change. He`s had mirrors shatter just from the heat of polishing.