The best strategy I've found for engie (That doesn't use minisentries for evil purposes) has definitely got to be the Rescue Ranger, Short Circuit/Wrangler (Depending on situation), and the Jag. That way, you can quickly set up your level one sentry close to the front lines, keep it repaired without having to worry about wasting metal that you want to use to upgrade your turret, and also, in the worst case scenario, keep a soldier or demoman from destroying it just long enough for it to build up.