I can buy a 3 inch '70's model 36 .38 special.  It is in very good blued condition. Wood handle and squared grip.
1) First of all can it shoot plus P's.  
2) Can you have a smaller barrel put on it? I'm interested in it as a backup
3) What is it worth? Thanks

1.  I am pretty sure that all 36's are rated for +Ps but don't take my word for it.  It is the older airweights that are not.

2.  Re-barreling a S&W is EXPENSIVE.  I looked into this a few months ago and every price I could find hinted that you would spend around the price of the gun for the new barrel and installation.  

3.  Prices on these things is all over the map, I have seen as cheap as $200 and as high as $400.  I would not pay the high end price, but the lower price and middle ground are good.  I would make the call based on the fit of the gun and the trigger.  If the cylinder has much play at all pass.

shooter

"Book value" is in the $200 range in very good condition, but I see these bring considerably more than that all the time.  Remember, the 3" and square-butt variations are long-since discontinued (1991 for the SB and 1994 for the 3", IIRC) and will usually bring a premium.

FWIW, I passed on one at $265 last year (excellent, no box) and have been kicking myself ever since...

It can probably handle some +P's now and again, but I wouldn't feed it a steady diet of +P's unless its specifically rated for +P.  

The 3" guns are a bit rarer than the 2" or shorter guns.  Is this a 3" heavy barrel or a 3" pencil barrel?  The heavy barrel may increase its overall value (cost to you).

$200 is a bit low for what I've seen recently on 3" 36's, from what I've seen online and at shows I'd figure between $250 to $350 depending on condition.

Hope that helps.