Auto Transmission problems

I have had a '98 SL2 (odo=123K) for about 16K miles. When I bought it,
the seller said that he put in new auto trans fluid, which I checked. I
didn't worry too much about it.

So yesterday I am in slow-n-go traffic on the going-home commute, and
then suddenly find I can barely get off the interstate. Don't have
really any trouble moving, but it seems not to want to go into 2nd or
maybe 3rd. Once the car is parked, I can only guess that it is a low
fluid level (sort of hard to tell on the dipstick). I put in 4 quarts
(Dexron III), indicating it was BONE DRY, then I cross my fingers, start
the car, move the shift through P/R/N/D/2/3 several times, then take off.
I didn't notice any problems going home, and it seemed even to have more
life to it.

Drove off this morning, buzzed 40 miles down the interstate to work,
thinking, hoping the worst behind me. Get off the freeway to the stop
sign on the off ramp. From the stop sign, I can get it in first, but
it's giving problems shifting to 2nd---revving high. It kicks in
eventually. But not after moving at 10 mph for several hundred feet.

I will go to dealer or transmission shop. Is anyone willing to guess
about what should I expect to hear? I have seen posts here that I should
not take the first opinion I get. Is that true? Or am I beyond the
luxury of choices now?

Oh, I forgot to add: this problem with the car now must be a serious
problem, because I am tightly budgeted for the next 2 months, and
spending it on major auto repairs is not in the budget. That is, major
car problems happen at the worst possible times.

Thank you