Are you sure that the spring seats are already gone, and not the end links? Just turn the wheel to the left and reach behind the wheel and grab the thin verticle bar and try to shake it. If it moves, replace both sides. These will rattle a lot on small bumps.

A good spring compressor is all that is required, and a little patience as tightening/loosening the spring takes a while. Do get new nuts for the 4 bolts that hold the strut in place, these are 'stretched' and probably shouldn't be reused. On the other hand, the top 6 tower nuts I always reuse.

When lowering the strut out of the car, be very careful of the ABS wires and the sensor.

You will need an alignment. Before you start, find a local shop that the small repair shops use. Not a tire store! Depending on where you live, it costs around $100 and they will also check the steering arms and other bushings.

BTW, shocks are usually good for 120K in normal driving. But it depends on the number of potholes or washboard roads.