FWIW, I think pressurized portafilters are the devil

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I have heard several people mention that they ordered a Saeco non pressurized portafilter direct from Saeco USA for right around $30.
http://www.saeco-usa.com/
My brother has the Starbuck's Barista, and I think it is a decent machine to start out with if you can find a deal on one. He has the Starbuck's (Solis) grinder as well. He only uses the non-pressurized portafilter and currently has a cheaper 49 mm metal tamper that he uses to tamp the coffee... it works, but I know he wants a proper tamper.
I would caution you to be sure all the monkey business in the bottom of your modified (de)pressurized portafilter is cleaned... I think it would have a tendency to traps lots of gunk and possibly go rancid.
As for the tamper you have, it should work fine. Do a Staub style tamp (NSEW). I know one of the problems with Saeco machines is that it might take a 52 or 53 mm tamper depending on the baskets/machine/date of manufacture.
Report back when you get your Mini, I think you will be pretty happy with the Barista for awhile longer.