Heat is the worst thing for any engine so the most important gauges are those that can provide a direct indication by the numbers that something is wrong...so the 3 most important gauges to get are in priority order:
1. oil pressure gauge - Fords stock gauge will read normal as long as there is 8 psi being produced...however these engines require a minimum of 25 psi...so though the stock reads "in the middle" the reality is the engine may only be making 8 psi and you would never know you are in an oil starvation situation.
2. water temperature gauge - the stock gauge doesn't really tell you how hot you are really running.
3. wideband air/fuel ratio - definitely needed to let you know when you are running lean under a load.
Any other gauges are of a lesser importance compared to these three gauges listed above.