I've been asked about my Coverhem machine, so here is a bit about my machine and process.
I have a Janome Coverpro 1000 CP. I must have had this about 10 years and it is now discontinued. So it might be available inexpensively second hand.
I think the replacement model currently available is the Janome CoverPro 2000 CPX.
I have a Janome sewing machine and an older NewHome overlocker (which is an old name for Janome apparently) so I went with Janome becuase the price was OK and I like my other machines. I have not compared it with anything else.
I really struggled to learn how to use mine as I did not buy it from a dealer. However there are more resources available now, youtube and online discussions showing how to use coverhem machines.
I don't think these were available at the time, or maybe they were but it was pre broadband, anyway the learning curve was quite tough for me so I didn't use it a lot to start with which with hindsight was a real shame.
The Janome Coverpro 1000 CP has a unit on the back to hold the thread spools. I have damaged this on mine, and after the second time of dropping something heavy on it by mistake I removed it and use freestanding thread spools instead. This works very well. I understand from my local repair guy that this is quite a common issue. May be worth checking this if you are buying a second hand one.
I generally use mine with just 3 spools of thread and 2 needles, as I had a lot of problems with the middle needle in. I also flat complete hemming and then construct after as it didn't do well sewing over lumpy seams. These issues may be operator error. I like to put a little snip at the fold line through the overlocked seam and fold the two bits in different directions, this makes it less lumpy and the machines copes better. I don't have any attachments for the machine, though I'd love to do binding with it as they seem very expensive.
I only use my machine to hem knits (and anything else very stretchy) and I really like it for this as it gives a RTW finish on knit hems which I really like. If this isn't a big bugbear for you or you don't sew a lot of knits it is probably not worth having a separate machine for just that task as you can get a similar finish using a twin needle on a regular sewing machine. I like to have the separate machines set up for a single purpose and then sew lots of things so it works really well for me.
I normally use matching thread for all my projects which means, 4 on the overlocker(serger), 3 for the coverhem (2 needles) and another 1 + bobbin on the normal sewing machine. So I do buy 8 spools of the colours so I can get that match. This may not be something you want to do, so again that may influence your decision.
Hopefully this helps a little. Thanks Ruthie