Try an honest rogue
Most rogue abilities, enhancements stop improving after level 12
So the build is 13 levels rogue and 7 levels paladin -- a mechanic knight of the chalice (double specialized multiclass)
Could also work with 12 rogue 8 pally if you want higher base attack and another lay on hands at the price of a rogue feat and a die of sneak attack.
The 13 levels of rogue allow for 2 rogue feats (at 10 and 13)
The umd allows you to raise the cleric in the group to avoid party wipes with a scroll of raise dead. As a blend some people may think the rogue skills wont be high enough, specialized as a mechanic II will help.
The 7 levels of paladin give the staying power and saves at the expense of some sneak attack damage. The exalted smites (5 I think and they slowly come back), divine sacrifice, and knight of the chalice compensates for some of the lost sneak attack damage. The lay on hands (2 of em for about 150 htps each) and unyielding sov give large amounts of healing when needed. Also as a pally you can use healing wands (no UMD needed).
High charisma for great saves. At the 10th level of rogue take improved evasion to deal with the occasional roll of a 1 on a save while standing in a trap. At 13th level rogue take slippery mind to deal with will saves.
The base attack will also be higher than a pure rogue
Ability to range and melee is balanced because a mechanic adds intelligence to damage when using repeating xbow (a free feat to mechanics) and the sneak attacks blend well with pally click attacks (smite, div sacrifice, etc)
Stats
Race Dark Elf --- lends itself to rapier and short sword
Str 12
Dex 16 the last stat point can go here for better to hit if you like
Con 12
Int 16 lvls 4 and 8 add to intelligence for rogue skills
Wis 8
Chr 16 lvls 12 and 16 for better saves and pally powers
Feats --
toughness (for a less squishy rogue)
weapon finesse (dex will be very high)
force of personality
two weapon fighting
improved crit (slash or pierce)
rapid shot
skill focus disable device
as an elf and a rogue you can add 4 dex
as a paladin you can add 2 charisma
The pally also gets enhancements to an aura that increases your saves
All saves will be over 20 around lvl 12 and past 30 towards end game content
Ref saves vs traps around lvl 12 will be like +40s so you would only fail on a 1
Skills to focus on are spot, search, disable device, pick lock, hide, umd
Throw some into jump, tumble, etc with the spare skill points
The force of personality feat goes well with slippery mind