I would like some help on to knowing what I can host without crashing. Below I will list my PC and internet specs, and further down I will list what I actually plan to host.
Anyway, onto the PC and internet specs:
Internet:

PC (Main): AMD AthlonX2 3.0 GHZ Dual core processor
4GB DDR3 (1333)
PC (Old): AMD Athlon2 2.4GHZ Dual core processor
4GB DDR3 (800)
I plan to host a server with about 50k bricks, and somewhat nasty amount of events. It's a challenge, but not just jumping over blocks challenge. You have to solve puzzles, clear areas with automated events, as well as encounter a few timed events. It also saves progress, so the loading and saving variables probably will add up when a ton of players are in the file.
And for the record, I'm not interested in any hosting services. I'm sure if there are any problems, it will be most likely be on the CPU end, and that I can fix very easily but just getting a new processor that can handle more. However, I'd rather avoid this if possible, as there really is no other reason for me to upgrade my computer or build a new one, and it would just feel silly to do so for hosting a server. As well as, I would love to be able to still use my main computer, and just fire up the old one to host something.
So, please just tell me a rough estimate what the limitations would be for my setup. I'm hoping 30 players, but if more, then I'd really love to know, but I don't want to risk pissing people off seeing as this is a challenge and it would really suck to just watch it crash when you just got past a really tough section.