1. No, should be straightforward.
2. You need to make sure it can get enough air, if your case is MicroATX (about 35cm high/deep) and the old PSU is a 130-150w job that extends about 150mm into the case, installing a bigger ATX version will restrict the airflow round the case and into the PSU.
Any PSU you install will fail sooner if it's airflow is restricted, but the smaller MicroATX units can't provide enough wattage unless you hunt (and pay over the odds) for a high capacity one. I had a similar situation, if you really need a `full size` PSU, find the shortest DVD drive you can, and fit an extra case fan to draw air from the back of the machine.