I always like to put the O/S on its own drive, but only bc it isolates it from other system use, makes for cleaner backup & restore procedures, and keeps I/O swaps off the app drive/s.
Regarding O/S swaps, you can tell Windows not to use or allocate a swap file, which is what I would RX. If you have to buy more RAM memory, so be it (unless you're already capped out, which eliminates this option)
Samsung's Rapid Mode can be helpful, but only if you're going after the same data again and again and it isn't changing between fetches. That's true for ALL caching.
Larger SSDs are almost always faster bc the controller is throwing pieces into more and more buckets, which running in parallel, speeds up the process based on how many buckets there are to receive those pieces. This is not usually more than a 10% gain, but read the reviews.
Samsung Pro drives, unlike Eco, don't depend on caching for their performance, so on sustained Writes they are much faster. They also last longer, but that's not usually an issue, especially if you've turned off O/S swapping. I've managed systems many yrs ago where 99.8% of all the system I/O was page file swaps by the O/S. In that case, the single act of putting the O/S on it's own dedicated drive tripled overall system performance. I doubt you'd see anything like that performance improvement with any modern PC system, but it's something to watch for.
Once you shut off page file swapping, the O/S doesn't need a fast drive. Once up and running, O/S file loads are rare in this age of monolithic O/Ss, and they're mostly just large sequential reads anyway. (page file swaps are NOT, they're random I/O). Microsoft put simultaneous, overlapping, asynchronous file I/O in Windows in 1999, so I/O happens in the background all the time in little snippets if developers are smart enough to allow it. Surprising how many developers think loading up a gig of disk to a RAM data structure all at once is good practice. SMH. This is one case where the O/S gets it right.
Always buy the next gen tech when upgrading old systems so it will still relevant when you upgrade the rest of your system.
At any given point in time there's a sweet spot for SSDs (and most other things as well) where you get the most performance & capacity for the least per-capacity cost. This usually is a good tie-breaker when trying to decide how much to buy of what.
Samsung's quality is top notch. It's won more awards than you can shake a stick at, and the Pro series has massively outperformed its rated lifetime of I/Os. If that changes I'd consider something else, but until then, Samsung is the one to beat. (not much of a premium for their quality either)
If DCS were a database app I might recommend a RAID system, but from the comments above (with the possible exception of servers) it's not going to make any difference.
Cheers!