OK, worked chandlery for 10 years so here is the full speel!!
First you need to think if you ever want to pu an anchor winch on and if so buy rope and chain to suit your future winch as otherwise you might be buying all over again!!
A winch for that size boat would take 8mm short link chain and 14mm 8-plait nylon rope which would be spliced together. If it was me I would be looking at 15-30m of chain with 70-100m of rope behind it. These lengths would vary on what you plan on doing with the boat and where.
If no winch is planned then you can use long link chain but the other advantage of short lonk is that you get more wieght (better holding) for the same length. If there is no winch I would go 10mm chain and 16mm rope just because anchor chain and rope is something I like to go overkill on!!
I always prefer a 8-plait rope over a 3-strand as it doesn't hold twist as much and tangle in the locker. Nylon is good but can get hard with time so look at some of the modern polymide 8-plait or 12-plait ropes. The 8-plait is easier to splice but I recently had some 12-plait that was too hard to back splice so I experemented with a brummel locking splice and it worked a charm!!
