speaker and interconnects, how nice do I need

Make your own, make your own.

I would go SUPERcheap - the cheapest you can possibly buy. A few bucks. Then try more expensive cables, and see if they make a difference. You may find that cabling is the least cost-effective way to improve your system,

For line level interconnect, I use pro microphone cable, Canare StarQuad L-4E6S with ETI connectors. Mogami W2534 and ETI connectors is a good alternative. For speakers, I use Goertz. About $200 all in for two interconnects and two speaker cables.

Just one caution - if you are soldering teflon jacketed wire, be very careful to exhaust the fumes. Mogami is probably a better choice for that reason, but I have a soldering station with a fume hood, so I can get away with it.

Works for me, and my system is far from cheap. I would rather spend money elsewhere.

Before trying anything else....order some Canare 4s8 speaker wire and make your own cable. Super inexpensive and I was impressed by how good my little Dynaudio Dm 2/6 sound w/this wire. As far as interconnents, make your own. I made some using Mogami 2549 wire. Recently bought some Wireworld Luna7 and I was impressed how good they are for $40 a meter. Lots of choices and good luck finding the right combination! Bill.

Buying used is a great Idea. The cheapest speaker cable I have used is 12ga. jacketed in wall cable for ~$0.50/ft..  That and Inexpensive interconnects can be found on Ebay. This would get you by while exploring the market.

A pair of 10" Audioquest Type 4 cables could be had for ~$65.  Check out Ebay also. 

The Canare cable may serve you well. The 4s11can be had for ~$1.50/ft and the 4s8 is less. I bought 20' of the 11s however never put them into service so can't speak to how they sound against the Type 4s. Guess I need to do that. 

Music Direct was selling Audioquest Diamondback interconnects for $75. Can't say if they still are. I bought from them a 0.5m pair sold as demo for $45. Check them out online.

I second JW Audio. I have his interconnects and love them. His speaker cable is ridiculously cheap.
Also consider DNM and anticables.

I concur with the recommendation to start with Canare 4s8 or 4s11 as a baseline for further exploration, if you feel you need to. Buy whatever length you need from some place like https://www.markertek.com/. Don't bother with connectors at first, just twist together the white and red conductors in each cable and connect the bare wire to your amp and speaker terminals. Listen for a good long time and only then begin to explore if you feel it's necessary, or interesting.