The website is plain and simple and might come across a bit shady. It is entirely legit however, and I recommend you check the video and requirements before making a purchase.
You can buy the software through the icons in the top right corner. The software costs $20 and gives you a life-time license. There is no DLC involved and the software is updated regularly.
Upon purchase, you receive a link via email that will lead you to a humble bundle page. Here you can download and later install new updates to the software from as well, so save it somewhere you can easily find it!
Now download the installer from the humblebundle page in install the software. This shouldn't take long as the software is very small in size.
Check if the installment proces went as intended. You will be familiarised with the software in the tutorials videos!
Below you can find the first tutorial video. Go check it out, and you will be proficient with Dungeondraft in no-time!