Using a heatgun is time consuming and back breaking. At medium heat setting, you hover the heatgun 1 inch above shirt at a speed of about 1cm per second. If you hover the heat gun from left to right starting witht he upper portion of the image, after reaching the rightmost edge of the image, you move the heat gun about 1cm down then hover from right to left, and so forth. Slower with lighter and/or thicker inks. You will need to do wash tests to see if you get the speed correctly.
An oven should save you more time with less neck aches, upper-back aches and elbow pains. Search yotube for youtees and you will find in one of the youtees series, where the shirt is cured in a closed oven for 30-40seconds I think. Again, you need to do your wash test to see if you have cured properly.
A flashcure and heat press are also alternatives but never tried them. I have actually tried a borrowed heat press but did not have enough time to get the proper time. I use a heat gun/500w halogen lamp.
I think you should also try search for second hand dryers in your area to see how much they cost.