Monks are extremely good defensive tanks, no other class gets AC that good. You can't wear a shield and have to keep your body armor slot empty, but the dex bonus isn't capped by armor and spells like spirit armor work, you even get extra bonuses to AC for wis and monk levels.

Full plate allows only for +8 for armor and +1 for dex. Even a level 1 monk with 18 dex and 18 wis and the mage armor spell cast on him has 4+4+4=12 AC, more than the best magical full plate and the bonuses will improve with levels (more dex with cat's grace, more AC bonus with spirit instead of mage armor, more wis for raising at level up and wearing stat boosting equipment, AC bonus every 5 levels).

Besides the AC you monks get many other defensive bonuses (best saving throw progression, evasion, spell resistance, ... )

The main monk weakness is the offense, just like priests and rogues they only get 3 BAB progression every 4 levels and their damage output with fists isn't higher than with a staff or greataxe most of the time. Stunning blow and quivering palm can be powerful, but require micromanaging and can only be used a few times per day.

If you take a monk keep him pure class, BAB from other classes destroys their unarmed BAB progression and they get their bonuses at high monk levels. I'd take an half-orc with 20 str, 18 dex, 16 con, 1 int, 18 wis, 1 cha and raise str or wis at level up, if offense isn't important for you you can take another race, less strength and more con.