1999 Isuzu Rodeo

Greetings, and thanks for reading this post.
I just bought a 1999 Isuzu Rodeo for my stepson to drive to college.
It has transmission issues. When you put it into gear, it hits pretty hard, and I was told it only goes into 1st and 2nd gear. I test drove it a short distance, and believed what I was told.
I bought it fully expecting to put in a replacement transmission, but after talking about it to some mechanics, and reading your discussion groups, I'm now wondering if it might just need fluid added. And if so, should I go ahead and replace the filter.
What are your thoughts, and thanks again.
Bradley

I guess it would depend on the condition/color of the fluid?

Hey thanks.
I was wondering what you thought about it "hitting hard" when it goes into gear, and never making it to 3rd or 4th.
Do you think adding fluid will fix that? Maybe?