Nucleus, is there a better alternative?

Certainly the Nucleus can be beaten, many custom builds will have faster processors, superior RAM than off the shelf items. NUC's can be pretty off the shelf or more diy. Actually, my preferred choice for bang for buck is diy custom build. In very near future will be auditioning Taiko Extreme clone build, major attention to detail with every single part and total design.

Is the optical connection better than ethernet and will eliminate noise? 
I like the converter option and wonder what converters, ie. Fmc, are better. They seem pretty cheap.

I did notice that fmc require a power connection so this means more noise and hopefully this doesn't create a problem, but then again noise is not relevant once the signal hits the optical path.

I’m in a similar boat, had to stop short of completing my digital chain in the last few months but am in need of a server. I’m currently playing files: Macbook (w/ external drive) > Airplay > Airport Express > Mini Toslink > Bricasti M1se w/ network card … I want to run a server to the dac over ethernet, was thinking about getting a Nucleus to throw my files on (about 5tb) and I also use Tidal/Spotify … but on the fence between that and NAS … K41 seems to get a lot of accolades? 

Optical better than ethernet, no mass consensus, much variety of opinion. Unless you have optical internet provider, ethernet somewhere in chain, wish I had choice so could compare full chain of both. After fair bit of experimentation, prefer ethernet prior to server, optical after.

Those are DC power connections to ALL FMC, provided by switch mode power supply (wall wart) or LPS, linear power supply take AC, convert to DC.

Important point many don't think about, optical only eliminates the noise at point it enters FMC, DOESN'T eliminate noise emanating from WITHIN any equipment, cable PRIOR to FMC. This means you don't gain something from using optical vs ethernet, losses due to this noise the same. Theoretical advantage is you're not adding the possibility of more noise via optical cable vs ethernet cable, whether optical or ethernet devices  used are noisier is highly dependent on particular devices used. . Generic FMC are noisy devices, even with full galvanic isolation, losses are losses,  whether you hear this noise or not depends on system resolution. Ethernet devices subject to same noise limitations.

Sonore OpticalModule is an attempt to address this FMC noise issue, I have one on order, waiting on redesign due to chip supply issues.

@au_lait NAS vs internal storage, same deal with wide variety of opinions. Internal storage creates more noise within server, should really have separate power supply, maximum isolation for optimal results. NAS has it's own noise issues and requires more ethernet cabling, also more complexity to set up and operate. My present setup with NAS powered by lps sounds great.