In reply to Captain Gear: Right then....deep breath....
Yes the 201k is an electric machine, I think it was originally made as a hand machine but they then added an external motor (double check any version has this!!). It only does a straight stitch but it really is a greta machine, I had much less trouble with it than my current industrial brother!. Try to buy from a dealer who will have serviced it rather than privately.
In terms of sourcing fabric... The first thing to know it that we have quite a rubbish selection of specialist outdoor fabrics available in the uk and you will often have to look to the USA or Europe to get the more out there ones. Good site to use are:
http://www.pennineoutdoor.co.uk/
http://www.profabrics.co.uk/
http://www.extremtextil.de/index_en.html
www.shelby.fi
Dimension Polyant VX21 fabric , never heard of this (sound like a US brand name) but assume it is some type of nylon with reinforcing mesh... this may be similar and is certainly good for back backs.
http://www.extremtextil.de/catalog/Gridstop-Nylon-Dyneema-Ripstop-210den-Ny... Alternativly you could try and 500d cordura.
Good quality 1" and 2" webbing - ideally in red, for the best quality webbing stay away from Polyester and polyethylene and look for Nylon, it has a much higher abrasion resistance and just feels much more classy. For some reason getting Nylon webbing in anything other than black in the UK is hard unless buying large quantity... try this
http://www.profabrics.co.uk/shop/show_product_info.php?id=WN225-09. It is only back but is a very good webbing. If you don't mind using a lesser quality webbing (polyester) then multiple colours are easy to find on ebay.
National molding buckles, this is just a brand name of a buckle manufacturers, good quality buckles are easy to find on ebay, ITW NEXUS are a good name to look out for.
YKK zips - I'm after the really thick ones, I get all my zips from here, if you want the really thick ones then you want the No.10
http://www.kayospruce.com/index.asp?selection=detailed&uid=17943&ca... Ignore the picture, for some reason it has the same pic for all their zips. You need to buy the sliders seperately. There not YKK but are very good.
Who makes good quality foam for back systems and shoulders straps? You want to be looking for a EVA foam for the straps, buying it in small quantities is not easy and bear in mind that sewing through anything thicker than 3mm plus your fabric wont happen easily on a normal machine. For the back system I would go for a closed cell PE foam. This place sells the PE stuff
http://www.efoam.co.uk/indexm.html I get all my foam from
http://www.paulamar.co.uk/foam/materials.aspx but they only sell in bulk.
What type of thread do I want to use? Bonded Nylon 60's easily available on ebay, you want to avaid the cheap stuff as it is not good to work with, you should be looking to pay abut 10 -15 for a 3000m spool.
What type of material do I want to use for valance and insider of the back system? Would coated nylon work fine? 2oz or 4oz? Hmmm not really my area of expertise but I would go with a 4 or 6oz depending on if you are after weight saving or durability.
How do I bind seems? Do I need a special machine? You dont need a special machine but it will make your life a 1000 times easier and the end result much more professional. Have a look at these
http://www.sew-europe.co.uk/folders.htm
Will a singer machine sew through closed cell foam and material? To a point. Anything more than about 3mm of foam + fabric will start to result in broken thread/skipped stitches.
Hope this is of some help.