Toxer.
Action, (or string height) is usually measured at the 12th fret, between the top of the fret and the bottom of the string.
On my Maton Messiah, which I use for standard acoustic guitar work(ie.fingerpicking) the string height at the 12th fret is about 3mm.
(Measured on the 6th string).
On my National Duolian the string height at the 12th fret is about 4mm.
Same on my spider reso (Dobro).
There's not a lot of difference really.
Some prefer an action of about 5mm for
slide,
however.
The same guitar can easily be used for slide, and normal fingerpicking.
('m guessing you meant 5mm, not 50mm on your guitar).
Also, neck relief is not the same as action.
Neck relief is the slight bend in the neck that allows the strings to vibrate in an "S'shape, which is adjusted by the truss rod.
Some cheap ply top guitars sound great for slide, and combined with a soundhole p/up, you will get some great blues tones.
I haven't heard many nice things about the Fender reso's.
Johnson seem to have a good reputation for a cheap reso.
Keep in mind that reso p/ups dont really capture the reso sound as well as a mike.IMO.
I agree with Allthumbs about electric guitars, they all sound great for slide.
Try a Fender Squier, they're cheap, and sound great.