Go vintage for sure, a nice no.5 1/2 is the plane you will use most, no.5's are tippy in comparison.
If looking at pics online, as said check for cracks.
Pick up a no.4 or two of them for as cheap as you can get, they are extremely abundant.
As for the no.5 1/2...
Aswell as said pictures to show no cracks, normally on the cheeks you will see this...
I would want to see three pictures of the plane showing the mouth not chipped, and a photo of the plane that shows how thick the castings are from either end of it.
If its thin it is likely that it has been lapped by a numpty and is a big convex mess to deal with, for one thing.
I made a video of lapping a plane since there was no videos demonstrating how to lap a plane correctly on youtube.
I mindlessly copied them in the past, as they all seemed to demonstrate the same technique
of lapping with a longer and wider abrasive on a surface plate.
What an eejit I felt like after making a big convex mess, never again!
To lap a plane correctly you must have an abrasive that is shorter and narrower than the plane is.
A flat surface needs two registration points like demonstrated in David Charlesworth's excellent videos on hand planing.
This technique is called stopped shavings and ensures that you don't end up with a convex board.
Its the same principal with lapping a plane, as in... you must have two registration points
compared to making a see saw and dubbing the edges of the plane.
All other videos I've seen demonstrated are silly, and only ensure a convex shiny plane sole
when all the ink is removed.
What you won't see is the test with feelers at the end showing the convex profile they have made.
Its a simple process to do correctly and will ensure far more longevity in your abrasive, as well as actually being quicker to boot.
This is because all the loose grit gets pushed off the edge of the paper, the second reason is
You are only taking the material that you want to be removed off.
I have a self adhesive sandpaper that is narrower and shorter than the plane is.
You can then get your regular cheap paper roll and slice it down the middle to make a pair of strips.
If there is a big hump then you cut a strip that will take care of this localised hump first.
Once you can get it close you can lay a full length strip down.
I'll say it again, this full length strip is still shorter and narrower than the plane is.
Check frequently with a straight edge in both orientations.
My video skills are terrible, but it shows the jist of it working a no.8 with a higgledy piggledy
sole.
Lapping done incorrectly leading to a convex profile can be very troublesome if you happen to have a very long plane that needs more than a lick done to it.
Is is also very troublesome if you have a plane like a no.60 1/2 which has a movable shoe in front of the mouth, If the bedding of the shoe is not parallel with the rest of the sole, then it will lift the plane if the shoe is adjusted.
Lapping incorrectly can lead to destruction on a plane that has been lapped incorrectly before and is really thin now.
You may well choose not to believe me, and proceed with someone else's method.
I think my thoughts will leave you suspicious of these methods though, and you might choose to
check with a straightedge before you go too far, instead of trusting the videos and waiting for all the penned ink on the sole to be gone and assuming that you now have a flat surface.
The likelihood is on a long plane, that you will create a convex surface before all the marker is removed.
Check the thing rather than trusting the ink!
If you are worried about creating a concave plane it would take little effort to get to flat
because of two things...
Lapping on an abrasive that is longer than the plane is will always favour the edges.
You are only removing a tiny area compared to see sawing on an entire area of cast iron.
https://youtu.be/3MlE7Nz3eKg
https://youtu.be/w_ux786ODwg
Tom