Items I have already a-
Behringer Eurorack 2642A mixing desk
Tannoy Reveal Active monitors
Pro Tools LE 6.7 with Digi002 Rack (with Waves Gold Bundle Plug-ins)
for mixing and production/mastering
Logic Pro 7 (for synths and MIDI sequencing)
Reason 2.5 (for synthesisers and drum loops)
Apple PowerMac G5 for home mixing/production
Apple PowerBook G4 for mobile recording
2 x Shure SM58 mics
1 x Rhode NT2 mic
1 x Rhode NT1A mic
1 x AKG880 mic
DVD player
CD player
Patchbay
Numerous guitar effects pedals
Numerous guitars
MIDI Keyboard controller
miscellaneous mic stands, cables of all types and descriptions etc...)

1. What other equipment would I need to complement the existing set of
hardware?

The usual - mics, preamps, monitors. Assuming you'll never record drums,
you need:
- a couple better vocal mics - higher end Rodes, AT4050, TLM-103
- a pair of small diaphram condensers - AT4041, MC012, Rode NT5
- some mics for speaker cabinets - SM57, MD421, D112
- preamps - Presonus Digimax LT (which will expand the 002R to 16 analog
inputs), FMR RNP
- monitors - start with Dynaudio, shop around, keep the Reveals as
alternates since you probably know them reasonably well

AKAI S5000 sampler (or equivalent)?

I'll give you one good REASON not to get this g

2. Would you recommend getting a decent mic pre as well? At the moment
I currently use the Digi and Behringer for vocal recording with
surprisingly good results.

Stop reading this, get in your car, go to your nearest pro audio shop, and
rent an FMR RNP. You'll be surprised how much you'll regret calling
Behringer mixer pramps "surprisingly good".

3. What about an ADAT machine? Only for compatibility if nothing
else...

Compatibility with what? A museum? Digital tape is deader than plaid and
paisley combined.

4. Anything else you can think of?

Firewire drive for the Powerbook, better EQ and compressors, Waves IR1, but
these are lower on the list than the above.