Maintenance on a 2000 with 55K miles

In March, I purchased a 2000 Avalon XLS with 45K miles. It now has 55K and although I am not having issues with it, I would like to perform a basic tune up. Previously, 1 owner vehicle (elderly woman who drove very limited, local only) and car was very regularly serviced, oil changed, etc.

One issue I am having is upon starting up, there is a very loud squealing noise which usually goes away after the car has been running about 15-20 seconds. Had all the belts replaced in July and it did stop, only to reappear about a month ago. Any ideas what this could be?

Also, even though there are only 55K miles on this car, it is almost 9 years old. Do you recommend I just go on and replace spark plugs, etc., or should I not fix what isn't broken??

Thanks!

Here's a snapshot of what maintenance is due. I would contact your dealership for a confirmed list: http://www.edmunds.com/maintenance/detail.html?year=2000&make=14&model=1074&styleId=1325&engCode=6VNAG3.0&transCode=AUTOMATIC&mileage=55000&zip=46350