I would recommended you change the ATF every 50k miles or so....that seems to be the consensus. Not sure if you can drain the torque converter on your 99....I can on my 98.

Like you, I shied away from changing the ATF and filter myself, even though I do it on my other cars. If you shop around, you should be able to do a lot better than 450, especially if you find a place that lets you provide your own fluid. While you're in there, you might consider replacing the plug adapter, which is prone to leaking.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 179k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 77k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife