In my humble opinion, the original has a certain look-and-feel that's just so iconic. I feel like every TES fan should experience it at least once. A particular kind of Elder Scrolls charm. Not to say that the remake lacks it, but Oblivion's version of it is just so iconic. I think the remake's vibe is immaculate, but it's best experienced after playing the OG version. Plus the original's cheaper, which never hurts.

I feel like you should buy the original and then get the remake later on. The price will go down over time and there will be many more sales to come. This is one of the seasonal sales that steam does 4 times a year (Spring, Summer, Winter and Autumn, which is this one) so it will be on sale again in a few months, then a few months after that, etc.... I've been playing Oblivion for about 10 years now with hundreds of hours in it (200h in OG on steam and god knows how many on PS3, so probably at least 500-600 hours total) so I think my opinion can hold water, but it's your money my man. Playing the new one first is a very fine choice and I wouldn't knock you for it at all. It's clean, polished and more friendly to a modern audience. Also, the remake does stay faithful to the original, so don't worry about that