Metric/Imperial Questions and Other Questions (New RepRapper)

Hello community,

I have been planning on building a RepRap (Prusa I1) for some time now, and I finally decided to dive in and buy some required parts after doing vigorous research. I am currently on vacation in the USA, and I return to the Netherlands next week. I want to get as much things needed before I head back in order to avoid shipping costs later down the line.

So far I have:

Threaded Rods (5/16)
Smooth Rods (5/16)
Skate Bearings (608Z)
Plywood Plates (1/4 in)
T5 Timing Belt (3 Meters)
RAMPS 1.4
Arduino Mega 2560
5x Motor Drivers
RAMPS cables
Old ATX Power Supply

I still need to get:

Printed Parts (Linear Bearings? ABS Bushings?)
5x 42lb Bipolar NEMA 17 Stepper Motors
Wade's Geared Extruder
Hot End
3x End Stop (MakerBot)

So here are some of my questions:

1. Are imperial nuts/bolts/rods compatible with metric printed parts?
2. I see some sets with bushings and some with linear bearings, what's up with this?
3. The hot end I plan to get is $60, is there a cheaper place to get one compatible with Wade's Geared Extruder?
4. I don't see a Z motor connector block on the BOM, where do I get it?
5. Total cost of my build will come to around $435, is that acceptable?
6. Is a "ballpoint pen spring" literally the spring I can get from a ballpoint pen?
7. Is there an easy way of converting METRIC (M8, M3, M4, etc) to IMPERIAL so I can purchase materials here at Lowes (USA)?
8. How much plastic does it generally take to print a full set of Prusa Mendel parts? I'm interested in printing them and distributing them for no cost (except for shipping and plastic cost) in order to increase the RepRap user base.

Thank you very much for answering any questions, I'm excited to build my printer and get it calibrated and going!

Pla bushings are plastic, and can be printed. They are part of the original prusa design.
They slide reasonably well along the smooth rods, or seem to on my half completed printer, but I can see how linear bearings are a worthwhile enhancement.
I'd guess bearings are better, but bushings are closer to the ideal of printing as much of the printer you can. It's up to you, but you have to be clear because they require different printed parts. 

I have yet to choose my hot end, but $60 sounds reasonable to me. However my current favourite after only minor research comes from the Netherlands
[reprapworld.com]

I live in the USA and I'm not sure why the interest in imperial dimensions. 3mm is very close to 4-40 but I think 3mm works better. Not sure what 4 is equivalent to but 8mm is very close to 5/16". I shop at Lowes all the time but they are not the source for cheap hardware like nuts and bolts. McMaster Carr is my go to for hardware. If your on the east coast shipping is fast.

Building a printer is something you should research and plan for but up to a point... then just dive in. All these detials will work out.