Always best to try before you buy. Since the amp is owned by a bud, that should not be a problem, and perhaps you have already played through it.
Play through the amp and really consider if you want to deal with the weight and long-term cost of keeping an all-tube head serviced. $700 for a recently serviced SVT in good condition sounds like a decent deal
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The amp weights 70lbs sans rack. A good flight case puts the weight in the 100lb territory. You can keep the weight down by using an injection molded case like an SKB, but keep in mind these cases are not proper flight cases. They can have problems with the rack rails pulling away when you use them with heavy gear. Also if you use an injection molded case to brace a heavy load in the back of a tour truck, the case can buckle under the load and allow the gear inside to get crushed...I know this from actual experience
. I warned them of the danger, but they did not listen and a very nice keyboard was destroyed.
A sextet of 6550s will run you about $270. Tube life can vary a lot depending upon how frequently and how hard you run the amp. Also it's recommended to let the tubes cool for several minutes after you are done before moving the amp.
Often when it's time for new tubes you can just throw a new set in and bias them up, but SVTs are fairly prone to taking out screen resistors when a tube goes bad, which means a trip to the tech, unless you can do the work yourself.
If one output tubes goes bad and it does not blow the screen resistor, it is possible to roll a matching tube into the amp, but you need test equipment to determine if the tube matches, and you also need some tubes to select from.
The first time you need new tubes, consider buying a full matched set with the same characteristics as the old tubes. Keep any good old tubes as spares.
Ideally you want a full matched sextet of 6550s. However this amp has two bias controls and the tubes are organized into two circuits of three tubes. So if one tube goes bad, and you still have five good tubes. You can also buy a matched trio; keep the two working tubes as spares. The new matched trio should have characteristics that are fairly close to the original tubes.
Tube matching characteristics: Some tube sellers just give you a grade. The grade may be a number or a color, but tubes are actually matched on two factors: plate current and transconductance. Many tube sellers put a sticker on the tube bass with ratings for these two factors. The problem is testing practices and testing equipment has not been standardized. This means if you buy tubes from two vendors with exactly the same plate current and transconductance ratings, that they are not necessarily going to match. I suggest finding a company you trust, and always buying output tubes from the same company.
Also it's a good idea to pay extra for burn-in services if they are not included in the testing and matching process. Burn-in typically costs a dollar or two per tube. During burn-in tubes are operated at working voltage for an extended time, like 24-48hrs. During this time many tubes fail. Also during this time the tube getter clears impurities from the tube vacuum. The tubes operating characteristics will shift until all of the impurities are cleared. After burn in the tubes will tend to be more stable, so matching will remain valid over a longer time period.
As far as whether this is the right SVT for you! IMHO if possible, play through an SVT CL and SVT VR. The SVT VR is a recreation of the vintage amps. The [SVT II and SVT-2 (non pro)] are fairly close to channel 1 of the vintage SVTs, but have a graphic EQ.
I believe 2-Pro is closer to a CL with a graphic EQ. These amps have a Master Volume and a more gainy preamp. The idea is you can turn down the Master so you are not too loud, and then crank up the gain so you get some drive from the preamp. Keep in mind that preamp drive does not produce the exact same sound and feel as pushing the output section into OD...but it may be useful to be able to get distortion at a lower volume.
Anyway, good luck with the decision
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