Welcome to the community, I hope you stick around . You can see my take on Blesta vs WHMCS here: http://www.blesta.com/forums/index.php?/topic/2512-blesta-vs-whmcs/?p=17790
When you buy direct you get support from Blesta, via the support desk, phone, etc. When you buy from a third party (re-seller, distributor, hosting company), they provide you the support, you can also get support from the community here.
BuycPanel yes, GoGetSSL yes, Proxmox yes, cPanel DNS / PowerDNS no / no, you can feel free to open a feature request, hire a developer, make it yourself or ask cPanel / PowerDNS to do it.
Yes you can sell VPS / Dedicated Servers, SolusVM / Proxmox, Virtualizor for VPS, Noc-PS for Dedicated or Universal Module (Manual).
You can use the free live chat plugin from PauloV, or you can just use a paid one like Zopim and put the code into the structure.pdt file.
Easy if you know how, you have all the code there for you to look at minus the 3 files for licensing which is encoded.
Patched when possible, you can look here: http://www.blesta.com/forums/index.php?/forum/16-news/ for the past updates, and there's hardly any bugs, just ones which people have missed in beta versions, etc.
Depends who you use, most companies work in the opening hours. Blesta is 8am - 4pm PST.
Yes you do require it for support and updates, you can get support from the forums or your vendor if you wish to, and you don't have to upgrade Blesta you can just patch the version you are on. And when you are happy pay and upgrade. You get the year from when you pay.