Fluid velocity is dependent on flow area - but you should have an idea as to the volumetric flow rate. The outlet temperature of the fluid can also be assumed or calcualted.
You need to obtain all the fluid properties and flow conditions first and then assume whatever you need to know.
Then decide on a HE design which in your case is basically a car radiator - ie fins/tubes etc
You will find that the heat exchange correlations are very much dependent on the HE design and flow behaviour etc.
So ask yourself
- what temperature do I need to cool this fluid down to?
- what is the flow rate of the hot fluid?
- What is the mean temperature of the air stream used?
- any corrosion issues that limit material slection and/or HE design etc
- design redundancy and cost etc
have fun