I think you are quite right, not a great deal of investment is likely to be needed. However, there will be ongoing maintenance and renewal (weren't the air bridges second-hand?) and I've a feeling STN's been getting tattier over the last 3/4 years while the uncertainty over its future was being resolved.
The biggest issue is the purchaser putting up £1,000,000,000 of someone else's money. That has to be paid for (interest + repayments) before dividends can be paid to the owner or profits re-invested in the facilities.
Ongoing maintenance happens every year so assume there is not going to be a huge backlog as quite simply losing anything mechanical will have its own implications on safety on a day to day basis and DoT would be involved very quickly.
Getting a Billion to buy will be surprisingly easy (crazy as it sounds) as despite there being a recession there is and will always be cash available for good investments with good long term returns.