Hi,
Sorry for answering that late...
Generally speaking, I agree with the guys responding before: judging on the sound of an instrument just based on reading the specifications is very difficult and risky. Also, the price is not really cheap, I would not risk that "experiment". I can give some general suggestions but would highly recommend to try the instrument. Since it is handmade, there could be the chance to even meet B&S and influence details (not sure if B&S is offering this).
I must admit that I never saw the Sarastro nor did I play any B&S Basstrombone so far. But reading the specifications, I would say that this instrument is a typical "German Style" trombone:
- It uses a conical slide, meaning the bore of the upper slide tube is slightly smaller than the bore of the lower slide tube. This brings the sound closer to the horn family, making it sound more full, warm and soft.
- It uses gold brass that adds warmth to the sound compared to classical "American" Trombones that are typically using yellow brass (gold brass sometimes on the bell only). Gold brass all over the instrument makes the instrument more durable because of the higher content of copper
- It uses a red brass bell that contains even more copper that should add even more warmth to the sound. This feature could make this instrument very warm even when compared to other German style trombone models. Some players say that a copper bell in combination with the conical slide could be easily "too warm", not sure I'm using the right expressions but words like "dampened" or "dull" are coming to my mind. Since B&S is offering this instrument in this configuration, I don't expect that this is completely unusable. But it might be close the edge between sounding nice and warm or dull. This is certainly a point where testing the instrument would be very useful. It is probably the only way to understand if it is OK for you or not.
My advise would be to make the decision for or against this instrument (or more general: Germans style) dependent on the kind of music music you intend to play. I see German Style trombones more suitable for classical or chamber music, brass ensembles etc. Here the full warm sound can fit perfectly and the instrument allows a high degree of virtuosity in small groups. I can only speak for my German style Laetzsch Cieslik vs. the Bach 50 (typical American style) I played before. With my Laetzsch I can form the tone much more individually which can be a blessing or a curse, depending on the person behind the mouthpiece...

The Bach sounds much more consistent between different players, the tones are locking nicely in and sound great - but less individual. Even with practicing a lot on the Bach I could never reach the sound quality of my Lätzsch.
But I must also admit that German style trombones are typically not very strong in projection. In Big Bands, marching Bands and any occasion where you need to play as loud as you can, it can be very hard for the player to "survive" against American style trombones. One reason for this is that the brass of American style trombones is much thicker than for German Style trombones. In fact, it is one major feature or even objective in making a German Style trombone to keep the brass as thin as possible.
Please note that the differentiation between American and German style trombones is not as clear and fixed as it may sound. Historically, also American trombone manufacturers, e.g. H.N. White (King) were using conical slides and even copper slide tubes and/or sterling silver bells to achieve a very warm and full sound. And also German trombone manufacturers like Laetzsch, Juergen Voigt or K&H are offering "American Style" trombones. I therefore think it makes sense to also look for the parameters of an instrument, not only on who was building them.
I hope this little excursion was helpful and please excuse my certainly not perfect English. If there are any questions, please feel free to ask.
best regards
Marco