What's interesting with this question is the shoestring budget. This makes it less off a professional server question. However there are some simple tests that can be applied to this kind of question regardless of scale of economy. (This is after all a site for giving / getting answers not comparing the size of each other's paycheck).
To directly answer your question. No SD will most likely be fine. Oddly a backup that completely overwrites the card is one of the least intensive uses of one. Its not the number of bytes you write, but the number of times you change an individual byte) What really kills them is letting a program use them for temp space (as some programs will automatically do the moment you edit a file directly on the flash drive). Remember that backing up every 5 hours is only changing each sector every 5 hours. It will wear, but after a long time.
Is this advisable? That's a simple question to answer:
What is your data worth to you / your company / your client?
Think of a backup like an insurance policy. On the assumption you're going to replace this backup solution completely in 2 years. $70 is cheap for protecting the average home business data for two years. The price of a sizable SD card these days is almost as cheap as many of us pay for a coffee. So only consider using an SD card if loosing that server's data would be an annoyance (couple of weeks work wasted) and nothing more.
Make sure your backup solution is verifying each backup (SD wear should be detectable if you also store check-sums of the files) and swap between two or more so that if one fails you're not more a day or two behind.
Finally consider using a good quality flash card. Remember that photographers fill up and clear down these daily. Your suggestion of doing so once in 5 hours isn't that far removed from what the good quality ones are designed for.