Many are based off of sources released here(80%+):
Downloads
Every RSPS is different, each one provides their own client(as soon in the above link), the only RSPS that provide the same client for multiple RSPS are the giant ones like soulsplit used to be(it would load a loader which had a pk server, a 718 server & a 317 server and u could pick one).
You would need a client developer who can get the memory usage down and patch memory leaks aswell as occasionally add client sided updates(like orbs, Remebering logins,etc).
You'll need a server sided developer to do server-sided updates such as(bosses, pvming, bug fixes, skilling, anything ingame just about).
I'm guessing you wont need a web developer(lol), I think u should use IPB forum's personally like version 4.1+ or whatever so people don't deface the forums if it grows, you'll also need webhosting with ddos protection & a vps with ddos protection, as the rsps grows u may need a dedi to handle the playercount to prevent lagging.
ALL Rsps typically have voting systems, heres one some use for example:
GamingToplist Voting script for any server
Voting is 100% NEEDED as it allows you to get higher in rsps toplists which lets u generate playercount.
Donation systems theres all kinds of, A popular one is rsps-pay.com.
In terms of server type, I would highly recommend a osrs server with just osrs items OR a 317 with merged items(as in chaotics, osrs items,etc) such as ikov(a huge rsps) had, one of those could be big, 614's and 718's have gradually all died off.
The RSPS market is gradually slowing down/flagging but quite a few remain, I'd say probably 1/4 of the RS playerbase is still left in RSPS(so 20k-40k)
If u have anymore questions I'll try to answer <3.