IDBA-UD assembly usage

[patouch74]

I'm trying to use IDBA-UD with metagenomic paired end data (2files R1.fq and R2.fq) but I'm not sure about the commands:

1/ first I merge them:
bin/fq2fa --merge --filter R1.fq R2.fq read.fa

2/ assembly ( I just want k=51)
bin/idba_ud -r read.fa --mink 51 --maxk 51 -o output

Am I doing right ?

And I have 3 outputs:
contig-51.fa
contig.fa
scaffold.fa

which output I have to use ?

thanks