Hello Clynton, it sounds like you're in for quite a "bonding" experience with your 550
. Since you're apparently not shy about doing some extensive DIY work, here are a few more things to consider if they haven't been done recently, or at all.
First run a good compression test and leakdown test to get an idea of the health of the engine, and "read" your plugs
Then here are some known areas that require maintenance/attention on the 550:
1. Replacing all coolant hoses, but especially those under the intake
manifold, with SRI silicone hoses. Also consider testing and/or replacing your coolant thermostat
2. If you do #1, which requires the intake manifold to come off, then you'll want to replace/upgrade the intake manifold gaskets.
3. Pulling the fuel injectors and having them sent off to be professionally cleaned and their flows tested
4. While the intake manifold is off, consider replacing all 3 of the coolant sensors on the coolant manifolds and the collector manifold. Two of these are cheap - $20 and are known to become dodgy with age, and they control your electric fans, and the third one is rather expensive > $200 and it is for the coolant temp gauge in your dash.
5. Replacing the fuel filters - and I'd have the old ones cut open if I were you and inspect the contents for rubber particles. The 456's and 550's and the V8's of the same era are having issues with the soft parts of the fuel system that live in the fuel tank. The ethanol in today's fuels cause these soft parts to disintegrate and they then travel through the fuel system, wreaking havoc, and sometimes overwhelming the fuel filters and go on to the fuel injectors and plug them.
6. If you have any fuel odor in the cabin or in the boot area, carefully inspect the plastic fuel pump covers as they may be cracked. There are aftermarket replacements in aluminum that should sort this once & for all. The fuel odor can also be coming from leaky gaskets on the 3 roll over valves on the top of the tank.
7. I'd also pull the front plastic serp belt & timing covers and have a look at the condition of the belts & tensioner bearings to see if there's much grease being thrown out of the bearings. Same for the bearings for the smaller timing drive sprockets down by the dampner. Also inspect the fences on these drive sprockets to ensure they're not loose - they're only peened on.
8. While you're down there - have a look at the front of the A/C compressor and see if there's much crud around the front of it, indicating a leaky front seal. The leaking seal will eventually drip enough refrigerant oil down into the alternator below it that both will need to be overhauled/replaced.
9. While still down there - have a look at the steering rack boots to see if they're wet or encrusted with road grime, indicating leaky seals.
10. Our Bilstein shocks are known leakers, so inspect them to see if there's much oily road crud on them. They're cheap to rebuild (at least here in the US). While you're looking at the shocks - have a look at the front ones to see if the actuators on top have "wound" themselves up. If so, carefully unwind the wiring, and consider some of the fixes that have been detailed on here and the other site to prevent the windup in the future.
11. Back up on top with the timing covers still off, have a look at the rear & front edge(s) of the cam covers, as they're known leakers.
12. Inspect the P/S line from the reservoir to the pump, as it tends to weep at the reservoir connection. There should be enough slack in this line that you can cut off the weepy bit and reconnect it, as a temporary measure.
All this is simply maintenance and remedial work that is often neglected - if you're interested in upgrades as well, there are several things which are easy to do and have very good benefits:
1. HID low beam conversion - The 550 Maranello's headlight pods are modular, and the projector assemblies can be removed and replaced very easily with HID assemblies having the proper optics, etc. I have found the HID upgrade on my car to be a great benefit as I simply don't have the visual acuity at night that I used to (I'm 62). I've detailed how to do this yourself in an HID thread here on FLife.
2. "Sport" steering ECU - Some find the 550's steering to be too light, so a colleague and I have developed replacement ECU's with different levels of P/S assistance, varying from stock, all the way to "no" assist at high speed.
We have several upgrades available in other areas as well, depending on your level of interest in modifying the car, such as: coil on plug ignitions, hydraulic front lift kits, cooling system upgrades, etc. Some are currently available, and some are still being tested.