I need an online calculator (or a very good guide) to determine
whether it is wiser to put some much needed maintenance into my car,
or to trade it in.

Here's a great article:
http://articles.moneycentral.msn.com/SavingandDebt/SaveonaCar/KeepYourOldClunkerOrBuyANewCar.aspx

When doing your math, keep in mind not only the purchase/interest
price of the new car, but also take into account the insurance price
and the property tax.  With an old clunker you don't need
collision/comprehensive (and if you desperately want it then it should
be dirt cheap)... with a new car the cost of that insurance is wildly
expensive.

I've never actually seen a calculator that does what you want it to,
but I hope that article helps.