Advice Needed - Should I try to work with what I have or just go all out on a new build?

Hey! New here. Hope I’m doing this right!

Recently I’ve taken interest in a home server, and using an old PC (Dell XPS 8700) made an unRAID server. Was having a great time with it, and at my work (a thrift store) we got donated a Rosewill Thor V2. It has 6 3.5" drive bays and another 6 5.25" bays, and got it for a great price. Problem is, the case is MASSIVE. Aaaand the cables from the original PSU are too short to reach the motherboard in the new case.

So I have 2 options at this point: buy cable extenders, and buy a fan splitter cable to power the 6 fan connectors that were in the case. On one hand this is cheaper, and would likely get my server back up and running quickly, but on the other hand, it’s a proprietary motherboard from 2014, that has 2 fan connectors, uses DDR3, has no M.2 connector, only mSATA, and overall just isn’t very compatible anymore. I think it also only supports 1GbE, which is all that I have but isn’t very future-proof in the case that our ISP upgrades stuff around here.

So my other option is, complete upgrade. Scrap (most) everything I have, and get a fresh start. I could get DDR4, 2.5GbE, m.2 connectors, more SATA connectors, and a faster and more future-proof server. Right now, if this is the path I follow, I have found an MSI X99A SLI Plus motherboard for 100 bucks, which has a whopping 10 SATA connectors, supports DDR4, has an LGA 2011-3 Socket, and is a seemingly much more capable board than what I currently have. I could definitely use another motherboard, but 10 sata ports offers me plenty of room to upgrade if need be, where as 5 or 6 might lead to me replacing drives with higher capacity ones, which I want to try and avoid for the most part. Buying a new motherboard also means replacing the CPU, maybe the PSU, and new RAM, which for a cheap bastard like me adds up fast.

Theoretically I COULD do both, but knowing me it probably won’t take long for me to spend money on the remaining parts if I just buy the motherboard.

The things that I was using on my server was Jellyfin, and really just the storage that was in it. I’d love to take advantage of all the things it does, like virtualization or ad-blocking, but I don’t think its worth going through all that effort when what I have is quite frankly, very janky.

I was hoping you guys would have some insight as to what I should do. Any questions and I will do my best to answer, any help is much appreciated.
(sorry if the formatting kinda sucks, it’s not often I use forums as I did not grow up with them!)