Most probably because that is his job and how he earns his money.
If you are confident and don't mind taking the sides off the case. Then the installation of addition memory and adding a new \ slave hard drive is simple.
It does need a bit of research, for memory, but there are plenty here to guide you through the process. For example, when you can determine how much extra memory you can add and what size hard drive you want ( this is for a slave, extra hard drive space. The actual fitting should take about 5 minutes.
The first thing to find out is the model of the motherboard and what ram you have installed at the moment.
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This will give you a good foundation in which to start.
It is also worth mentioning that motherboards do have limits as to how much ram that can be installed, but I doubt yours has been reached on a 3 year old system.