A metal detector will help with detecting embedded metal. The problem is it does not necessarily tell you where the metal is.The

Trying to repurpose old trim(base/chair/door/window/crown) is difficult. The paint can be way too expensive to remove in detailed trim. Old paint often contains lead---and planing/jointing it is not only dangerous, but illegal in many places.

What you could do is try and use old timbers from barns/houses. That would make the best use of those tools you mentioned.

A bit of metal can wreck a set of planer or jointer knives. You can use one of the hand held metal scanners (like the kind they use at security checkpoints) but as Handymac said, it isn't always easy to find the offending metal

Ditto on the paint.

It does not apply in this case, but a highly reliable source told me that the veneer mills use x-ray machines before they start veneering a log. Ceramic insulators apparently do as much or more damage than metal to very expensive veneer machines.

Thx, you all pretty much confirmed what I thought... so maybe my approach would have to be that I not pay much for the planer/jointer ... ? At this point I think I'm just cutting apart old doors and using the thick parts for table tops ... shouldnt be too many nails in those...

"shouldnt be too many nails in those..."

They WILL be there.

Count on it, and have spare blades available.

All you need is a broken off thumbtack to nick a planer blade.

The planer/jointer won't get ruined when you run into small bits of metal, so that's not really the issue. The issue is the cost of sharpening (or, in the case of disposables, replacing) the blades. You'll find that once you factor in the cost of keeping the blades sharp, old "free" wood is more expensive than new "expensive" wood. It can be fun stuff, but this is not a way to save money.

If you have adjustable blades you may even be able to continue working.
Just move one or two blades sideways slightly (not possible with some newer units with disposable blades) and the nicks in the blades are no longer aligned.

If you run into a nail sized piece of metal, the blades will be badly damaged and and there overall life will be significantly shortened by the time the damage is ground out during sharpening.

The blades are not painfully expensive anyway, but the hassle of stopping to install them eats up time.

Get your jointer with the Byrd head, it's only an $800 upcharge, and you'll never need to worry about a nicked blade.
Old metal isn't the only hazard; there's staples from the lumber wrap, and gravel from the bed of the delivery truck or your driveway, or garage floor. I have even seen bullets and buckshot in old wood!
Casey

Because our old house has yielded a plethora of old lumber that I have toyed with using, and I scavenge it to boot, I have asked about this in woodshops. One guy told me he has a set of blades he uses for uncertain prospects so he doesn't care about nicks.

I gave him one piece of painted flat stock to try out and he came up with a brilliant idea... he used a table saw rather than a planer. It was nearly an inch thick and only about 3-4 inches wide, so he ran it through the table saw on edge, cutting only wood but cutting the layer of paint right off. Obviously this wouldn't work on wider wood, but in this instance it was brilliant. Oh, and a little skill on the table saw doesn't hurt.

I think one of your best investments if you want to do this kind of thing would be a good paint stripping set-up - the infra red stripper is said to be good (around the old house forum).

Finally... the wood isn't all gorgeous under all that paint. We've stripped many of our fir mouldings and some of them are face grain and really not all that attractive. So be prepared to consider painting from time to time.

KarinL