I am on a very tight budget and would prefer a one off payment, and not be tied to subscription services
Linux + MariaDB (Fast MySQL).
Enterprise level throughput + scalability.
Cost == $0.
Well, cost included with whatever hosting you're using.
I run a private hosting company. Some of my machines are running 250,000+ uniques/hour + have been for months.
MariaDB can handle whatever load you throw at it...
Well... so long as your coders know how to write high speed code.
Most coders are clueless about this.
Tip: Pay for expensive developers up front + you'll save huge amounts of money avoiding rewrites.
We are just migrating from Insightly to Agilecrm.com. Take a look - it's fully featured and very good value. We are using it to replace Mailchimp and Campaign Monitor and FeedBlitz for some of our clients.
This is CRM database software - so may not suit your needs. But it's subscription; BUT it's low cost subscription.