To be honest, just about any of the current range of Pentax DSLR's will meet your requirement. The only exception is the K-500 as it is not sealed as you have wanted.
So, depending on your budget you have the K30, K-50, K-5II, K-5IIs and K-3.
The former four have the same mighty 16MP sensor, the latter has the new 24MP one. All will give you pleasure for years. For example I still have the K-5 mk1 and really not desparate to change yet, although I may get the K-3 in the next 6 months or so, if only to give the K-5 some breathing space.
Which ever Pentax body you choose, you will get a superb camera now, without having to explain why you chose Pentax in the first place.

I hope this helps.

Tough one.

I would probably say the following:-
K-mount manual focus:-
MX - Very tough basic Camera, with some features
K1000 - Cheap as chips, but built like a tank
LX - Pro model, but ensure it has had a full service.

K-Mount AF
Z-1p - Ugly as sin, but well made and reliable
MZ-5n, MZ-7 - The best of the MZ series models, and most reliable. Neither suffer from the broken cog issue of others
MZ-S - Yes it is another MZ body, but built like a tank and probably the best body since the LX. Rare and expensive.

M42 bodies
Spotmatic F - Has metering, auto on SMC and S-M-C lenses, a lever for other lenses.
SV - A classic, no metering, but a beauty.

Finally the ones to avoid.
MZ-50 - plastic lens mount, cheap build, cheap parts and will break! (I know!!!!!)
MZ-10 - Same.
MZ-5 - Earlier version of the "n", has the same cog issue as the MZ-50


Hope this helps.

Strangely, I have used my 50-135mm in Paris with the K-01. and although it is a big brute for the K-01, it was fine.
Still, all the small pentax primes are perfect for the k-01....

Various, well it will be more when the K-01 gets repaired, just got to find that ruddy recipt.......(Sorry Chris, I am still looking)

But the following are superb.

Super-Tak 55mm F1.8
SMC-Tak 35mm F3.5
Mamiya-Sekor 55mm F1.4
CZJ 50mm F2.8
All the above are M42 lenses
K55 F1.8
M50 F1.7
Tak 135mm F2.5
Tamron 90mm F2.5 Macro
F28 2.8
FA50 1.4
DA35 2.4
DA40 2.8

Oh hold on, all of my small primes.........

My K-5 has the same occasional issue. It seems to be a common Pentax problem.

I noticed today. Glad I got my DA*300 when I did, as it is over £100 more now. Looks like the FA77 is on hold, again!

One taken with the K55

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One taken with the M50mm F1.7

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Oh and another lens to look at for peanuts. M42 screw mount SMC-Takumar 35mm F3.5. It's a little gem

If you can afford both an M50 and the K55, then get both. They have their unique renditions and compliment each other perfectly.

One word about the 50mm F1.7 - Sweet!

There, said it.

As David has mentioned the 55mm F1.8 I can also say about that one - Pixie Dust!

As you may be aware, I have both and love both of them. Stunning little lenses.