a lot of m16 questions

1 how good is http://www.airsoftpost.com/product_info.php?cPath=25_97&products_id=28636
2 what do you need to switch a m16 to a m4, other than a different barrel and stock
3 how easy is it to work on internals of m4/16s

1. No idea
2. You'll need a shorter handguard, and if you're M16 does not have a removable carry handle, you'll also need a removable carry handle.
3. No idea

You don't really need internally to make a difference between a M4 and a M16. Depending on what upgrades were there for the M16 set up, you will probably need to downgrade the spring but nothing really major.
To work on a M16/M4 is hard to get the mechbox out some of the time, depending on the receiver. Once you get the gearbox out...its just a V2.

For metal receivers, take a look at this page: http://www.ebairsoft.com/airsoft-parts-body-c-68_24.html

the big questions are;
1 m16 to m4, do you need to modify the internals for the collapsable stock
and 2 is the metal body that I chose, good enough?

Well, for internals, I believe the M4 and the M16 have the same parts (for airsoft that is!) you'd just need to get a new barrel and anything else anybody mentioned, but thats all I see.

ah.... seems like what I thought, still wondering bout the metal body

I don't know much about metal bodies, but id just get an M4 one, I think your gearbox should fit into it, but id check with someone that knows a little more about that.

because this is my eventual rifle dream
so metal body on a Stag arms m16 then a scope paint and retractable stock

WW, just buy an e1 m4 of somesort then put an m16 upper on it and youll have what you want.

well, low on money and getting somthing m16 ish for the holidays so I am wondering about this...

To answer your questions in order...

G&P make great metal bodies, but I did hear criticism today, on how their bolt catch was crappy, and that pieces were flying out, but maybe I didn't hear correctly.

You'll need shorter handguards, and most M4s have retractable stocks. You'll also need a buffer tube whle your at it for the retractable stock.
If you do get a retractable stock depending on which one, you'll have to rewire it to the handguard unless you have a crane stock.

As for Internals...
Well actually I'm pretty sure internally, the differences of the M4, and M16 would be the M16 uses a non-ported cylinder...
Barrels pretty long, if it used a ported cylinder, it would not beable to shoot bb's smoothly..
Its not hard to take out the gearbox, and internals arn't too hard, theirs alot of videos to show you how to remove the internals, so if you get stuck, youtube.

ok, so the m16 to m4 stock;
rewire to front
buffer tube
that it?

Pretty much. But if you use a crane stock there is no need to wire to the front.

ok that pretty much answers everything