These are fantatstic engines, and are always a pleasure to work on, however a few pointers for the unweary.
As with all Honda Engines, The green oil pressure light should ALWAYS be lit when running, if it goes out you have no oil pressure. Always try and see an engine running to make sure it is lit.
Watch out for dodgy adverts - Look at the colour scheme of the outboard. Early models up to 1994 are dark grey with a white hood, Later models are silver all over. The newest are painted in the same style as new engines, with metallic 'Four Stroke' logos and Emissions compliance logos.
Hopefully you should have some servicing records - although these engines are extremely reliable and well made they won't work if they're not properly looked after. Check the oil to see it's colour and consistency, and check the spark plugs for oily deposits, oil burning is expensive to put right. Sooty deposits aren't such a major issue, and are usually caused by lots of use on choke or continuous trolling. (incidentally a 9.9 should have DR5HS plugs whilst a 15HP should use DR6HS, but if you're only trolling with your 15 put the DR5HS in, these numbers are for NGK plugs)
There is a commonly known trick about the 9.9 being uprated to a 15HP by removing a throttle restrictor, whilst this does almost work you should also know that by doing it you are only making it more powerful by making it rev faster, if you're not planning on going above 6000RPM all the time you won't feel any difference whatsoever. The 15HP version does also have larger main jet and a leaner idle mixture (for better high speed running) and a lumpier cam to make more power at higher revs. Removing the throttle limiter will therefore be of little benefit, but the 15HP can still only really be though of as a slightly tined 10HP, rather than a more powerful engine in it's own right.
If you already have a BF100 you may wonder why you should upgrade to one of these - well the answer is simply that the BF 100 makes 10HP at the engine, whereas the 9.9/15 is measured at the propeller.
Try and find out when the cambelt was changed last - they don't need doing often but they are a lot of hassle to change and can be expensive. Check the belt for frayed edges, cracks etc - if it's worn try and negotiate a discount as you will be looking at £30 plus labour to have it replaced. (Flywheel needs removing in order to do this)CDI units are apparently prone to failure, however I've only ever replaced 1 and that was due to user error, not a failure of the unit (12v connected to the stop cables, instead of being grounded hence killing the CDI unit). If you suspect the CDi, I was told that if you squeeze the CDI unit and the engine runs then it's the CDI, however I wouldn't put much trust in that theory!
It's always a good idea to have an engineer check over the engine - sometimes a service will be required. At our workshop a Service on one of these will usually cost around £120, but it is well worth it.