Obviously you don't need much of a generator to run a sump pump but you're not going to sit there in the dark listening to your sump pump run. So give some thought to what you might realistically use the generator for and size accordingly.

A battery back-up sump pump provides another level of protection, a second pump should the primary pump suffer a mechanical failure. You should consider having a spare sump pump and any necessary plumbing fittings to install it. Or have one of those "utility" type pumps that you can hook to a garden hose.

If all you want is to run a sump pump and maybe a few circuits, I would look at a portable generator. Very reasonably priced, good machines (e.g. Honda, Yamaha), and you can wheel it around and use it on other projects on your lot if you have a llarge piece of land and need power away from the house. I suggest spending the extra few dollars and getting a manual transfer switch - don't try to just plug things into the few outlets that may be on the genny. The only downside is that you need to store gas and be home when power goes out but that is probably workable. Just don't run the genny inside the house - locate a spot in a well ventialted area outside (down wind) and away from windows.

The standby units are nice, but they cost a lot more. I have a 12kW Kohler that runs off propane and has an automatic xfer switch. I must have power to run the well pump, refrig's and fans for my backup heat - we live in a remore area and lose power often. I was very close to picking a portable but in the end decided that given the load and the number of times we would need it, the standby was the way to go. I installed the entire thing and it cost 5500 for the genny, xfer switch, and materials for the slab and wiring. Having someone install it for me would have run around 8500. Not cheap.

You can probably get a portable for a few hundred bucks (small Honda at Home Depot) to under 2 grand for a really beefy portable Yamaha. 300 bucks for the xfer switch (plus installation), some gas cans, and you are good to go.

I know a numbver of people who live around me that have portable genny's and get by for days without power just fine.