A HOB should be perfectly fine.
plants also do better with a finer substrate then what is usually used for undergravels, and a lot of times either have a dirt underlayer, or something like root tabs added to the substrate - stuff you wouldn't want water passing over/through.
I kinda feel that undergravel filters are a bit archaic, and date from a period where airstones were the most effective way to move water.
Anyways, if you are concerned about what biofiltration you may be loosing without an undergravel, the plants will absorb some of the ammonia directly and altogether bypass part of the nitrogen cycle. The faster growing the plant the more ammonia it will absorb.
Glad to hear you are considering a planted tank. I'd recommend starting out low-tech, less equipment to buy, less maintenance, and (IMO) less stuff to potentially go wrong. Granted, the growth will be a bit slower, but it also gives you a bit more time to notice/correct potential problems and you spend less time trimming and pruning.
Poke around on here a bit, take a look at some of the stickies - there's some great info on this site, and a lot of people with a good deal of experience.