Here are your parts
Replacement parts for WHIRLPOOL RES7648KQ0 | AppliancePartsPros.com

Here is a wiring diagram
https://www.servicematters.com/d...0Sheet%20-%203406692.pdf

Here is a good site with basic dryer repair info
http://www.applianceaid.com/dryers.html

A job for owner?
Depends on how handy you are and how much effort you want to put in.

Need new thermostat?
This is a possibility. The Operating Thermostat has a small heater built into it, on lower heat settings this heater is turned on. This adds heat to the thermostat, it turns off more often and this reduces the dryer heat.
So if it is open you get only high heat.
The thermostat's heater also has a variable resistor (cycle switch) in series with it, this varies the heat output of this heater.

First check your vent system
Undo the dryer from the exhaust
Run a load
If the unit now has low heat then you have a vent problem.
Clean/check the vent system. Make sure the outside louvers open fully.

If you do not want lint in the house.
Take a pair of pantyhose, put one leg into the other and attach this to the dryer exhaust.
Leave enough room behind the dryer so it can blow out like a windsock.
Run a load.

Both the above will allow you to check the air flow and temp.
Also the above is NOT recommended for a gas dryer due to possible carbon monoxide problems.

Unplug the machine for the following;

Next remove the wires from the main heating coil. Measure with a meter from each heat contact to ground. Should be an open circuit. This is checking for a grounded element re: it has sagged or broken and is touching the case so it is always on. You could remove the heater assembly and check it visually.

Next remove one wire from the thermostat's heater and measure it's resistance, Should be approximately 7,000 ohms.

I would suggest you purchase a meter. You can get a decent digital multimeter for under $20.00. You do not need fancy though it is nice if the leads are a couple feet long.
If it saves ordering one unnecessary part it has paid for itself and you end up owning a useful tool.
Most places will not let you return electrical parts so if you order it, you own it.
A couple things to watch when measuring ohms and continuity
1. Always remove power from the machine otherwise you could blow your meter.
2. Always disconnect at least one side of any device you are checking. This eliminates the possibility of measuring an alternate/parallel circuit path.
3. When checking for closed contacts and continuity use the lowest scale (Usually 200 ohms). Then try higher scales. This scale is 0 to 200 ohms so if the device you are measuring is 300 ohms this scale would show an open circuit which it is not, you are just measuring outside the scale's dynamic range.

There is a good STICKY at the start of this forum about it's use.