SSD has no effect on a change in the system speed, except that everything is faster when reading from SSD compared with HDD. The advantage of SSD compared with HDD is the same now or a year from now. The price could be lower a year from now. The maximum capacity available could be higher a year from now.
Suppose you installed the SSD now, and one year later, the system has some issue that causes "slowness". Well, what would you do? You can't just keep hoping for more advanced storage to solve the slowness.
The slowness is one separate issue. How fast your system reads files is another issue. Using a HDD doesn't cause the system to get slower over the years. The spin rate, read rate, write rate of the HDD doesn't change. If the HDD gets full, the OS gets slow. There are a dozen reasons for a change from fastness to slowness, but the fact that you are using a HDD isn't the cause of the change.
As for installing Windows 7, the SSD has the exact same advantages for using Windows as it does for using Mac OS. To install Windows 7, you need to give up at least 50GB to the Boot Camp partition. If the storage (whether SSD or HDD doesn't matter) had only 80GB out of 500 GB available before Boot Camp, the remaining available space would be too little for Mac OS normal use. You need about 15% available space for any OS to work at top efficiency at all times.