I agree with Mr. President here. No-perk donations rely on generous people, and unfortunately the Garry's Mod community doesn't have many of those. I also hate servers that have a pay-to-win mentality and give donators huge benefits, but it's simply not practical to run a server on ad revenue and the occasional goodwill donation alone.
Having donators be able to buy items that don't effect gameplay (rank tags, playermodels, PAC3 or pointshop items, etc.) is a good way to get donations while still keep a fair server.
Another idea that I've had (although I've never tested it and I'm not sure how fair it would be in practice) is to have a set of time-based ranks with perks, and players have the option to donate instead of play on the server to earn those ranks.
However one thing that I have to disagree with Mr. President on is the cost. I ran (technically still do) a server that regularly had (maybe still has, I'm not sure) a peak of 20+ players daily. The server rarely experienced any lag*, only when players deliberately tried to lag it by way of piling a bunch of weapons on top of each other. This server ran on a VPS that cost me $5/month.
Server hosts that specifically sell Garry's Mod Servers (charge per slot) are usually a ripoff. Last time I checked, NFO charged $17/month for a 20 slot server. While they might be running better hardware than my $5 VPS, it's certainly not that much higher quality as to justify the $15 price hike. Even if it was, why bother when a cheaper server can run 20 slots just fine.
To start out with all you really need is a small (<20 slot) server, not a dedicated beast that can run multiple servers. I strongly encourage you to host your server on a VPS. Sure, it might be a bit difficult if you aren't very technically oriented. But it's definitely worth it for the cost and just a better understanding of how your server works so you don't need to rely on third parties such as a Garry's Mod Server Host's support team. There are plenty of online resources for learning, for example there is a pretty good
tutorial on the wiki and if you host with Vultr (my server's host) they have their own guide for running a Garry's Mod Server on their VPS.
Anyways, that's it for my rant. I wish you the best of luck in your endeavors, and I hope you take our recommendations into account.
* I think it's important to point out that I was running a jailbreak server, and I assume DarkRP servers are more resource expensive due to all the props and just the usual larger nature of DarkRP servers. So maybe you might only be able to get 10 slots on a $5 server, but I'm no expert so I suggest starting out with a $5 server and upgrading if you need to.