Electric Mixers save you a lot of effort when beating eggs. When you have leftover egg whites, beating eggs to make meringue can be a useful solution.
They also help when you get a 'three handed' recipe. I hate it when the recipe wants me to add an ingredient while one hand is holding the bowl and the other is holding the whisk.
There are two good choices for an electric mixer: Sunbeam MixMaster 2594
and KitchenAid Artisan KSM150PS
The Sunbeam MixMaster is like the one your mom probably had in her kitchen. It runs under $100 and it will get the job done. It comes with it's own mixing bowl but will work with many different kinds of stainless steel or glass mixing bowls. It isn't as robust as a professional mixer but it is also significantly cheaper. The only downside is that it does not come with the whisk attachment for beating egg whites.
The KitchenAid is a professional style mixer with a professional style price. The bowl is taller but narrower which helps keep things in the bowl. Unfortunately, that also means that none of your other bowls will fit. The KitchenAid comes with everything that you will need including a pouring shield to keep the mess down when adding ingredients while mixing.
In either case, if you are mixing multiple parts of a recipe, you will have to clean the bowl and the mixing attachments. between the mixes. I usually consider that an acceptable trade-off to mixing by hand.