If I can afford it, I prefer to use Nylon 6 for the chassis and if I want cheap armour, I tend to plump for HDPE. Having said that, I have used HDPE for a chassis before and plan to use it for the first chassis of Drumroll II so I can afford to have it ready for the champs in September (will rebuild it from something more durable after that)

HDPE can work for a chassis if you have no major load-bearing points, otherwise it will deform. On the previous Kaizer, I attached my lifting forks to an HDPE bulkhead but it simply bent whenever I tried to lift anything, so I had to brace it with a length of box-section steel. I replaced the HDPE with a bit of polypropylene in the current version and got rid of the steel and the results were much better. There is still a bit of flex in the material, particularly if it's lifting something heavy, but it is an improvement over the HDPE. Replacing the polypropylene with Nylon would be a further improvement still.

Tapping into both HDPE and Nylon can be done. Many people advise using wood screws over bolts when tapping into HDPE but I've never had a problem using the latter. There are loads of tapped holes in the Nylon chassis of Drumroll and only some of them are beginning to slip in their threads; that's after three years and numerous times being screwed and unscrewed.

The machining properties depend on your definition of machining. I would say that if you're using hand-tools such as a saw (or jigsaw) or cordless drill, then HDPE is the easier material to machine. But if you are getting it machined by mill or CNC machine, then I would imagine that Nylon 6 would be the better material to machine (although I could be wrong on that).

Here are a couple of data sheets to compare the properties of each material, and also a page of data graphs:

HDPE - http://www.directplasticsonline.co.uk/p ... asheet.pdf
Nylon 6 - http://www.directplasticsonline.co.uk/p ... truded.pdf
Graphs - http://www.directplasticsonline.co.uk/graphs.html

As the others have said, if you let us know what your design plans are it'll give us a better idea of what would work better.