The 3 stats that matter to an Engineer are:
Ballistic: Increases ALL rifleman and grenadier abilities.
Wounds: Increases health.
Toughness: Reduces damage taken.
All other stats are an afterthought!
QFT. I found that my damage was significantly bolstered when upgrading my ballistic skill. Ballistic skill should really be a primary and wounds and toughness come in second.
As tempting as it may be, try to avoid putting points into weaponskill upgrades for renown levels. In my personal experience, I only use the spanner skill to prevent destro players from running away. I don't if there are additional melee skills available to engineers (I'm sure a little research would answer this), but that's what Ironbreakers are for anyway.