May be I am naive or too new in the VPS, but the host location is irrelevant for a retail broker, because you are sending orders to your broker, and not to CME/CBOE/etc directly. Your broker server may be who knows where! Unless your broker claims that their servers are colocated in Chicago and even better, if in the same datacenters where CME/CBOE/etc hosts their servers. AFAIK, hedge funds pay millions of $ per month to gain that kind of access.
For building a computer, a good place to start is by running your entire trading setup on your current computer and looking at resource usage in Task Manager. It's time to upgrade when the CPU or RAM nears 100%.
For example, here is the typical CPU and RAM usage while day trading ES using NinjaTrader's orderflow + indicators. This is from our basic 3.5 GB RAM servers.
I've had some issues with lag using NinjaTrader 8 - which I suspect is the main issue. Yet, not at one time when I've had these issues have the RAM or CPU usage been high. It's been under 50 % on the RAM and way, way below on the CPU.
Re-booting my system seems to be the only solution. Not always easy or desirable when you're actually supposed to be trading.
If there is excess CPU & RAM capacity on your machine, then it likely being caused by something specific in NinjaTrader 8. Could possibly be a custom indicator requesting historical data as that is a common cause of freezing up. Only thing I can suggest is to email platformsupport@ninjatrader.com next time it happens. They'll typically start by asking for your log & trace files. If there's nothing obvious there, then can go deeper into your specific workspace setup.

I've had some issues with lag using NinjaTrader 8 - which I suspect is the main issue. Yet, not at one time when I've had these issues have the RAM or CPU usage been high. It's been under 50 % on the RAM and way, way below on the CPU.
Re-booting my system seems to be the only solution. Not always easy or desirable when you're actually supposed to be trading.
Next time it happens, look at the HDD usage, and how much data is cached. NT is a HD hog, and the way it processes tick data is just gross.