with heavy dirt he is struggling

The T 350 T-Racer is a surface cleaner which can be used only in conjunction with matching high-pressure cleaning equipment from the Kärcher Home & Garden program. Compatible models are the K2 to K7.
The package includes two extension tubes and 3 x 2 pieces Hochdruckduesen, 2 of which are pre-assembled. The various Hochdruckduesen have different water flow volumes. The delivery differentiated from less than 5.8 liters / min until more than 7.5 liter / minute.
On devices system can be adjusted by turning handle the cleaning pressure for a so-called bypass. So also sensitive wooden terraces can be cleaned from rugged surfaces like tile or stone.
Our Kaercher was rarely used in recent years, as I partially see him as bad design. While I get so the granite slabs on the patio and cleaning the paving stones in the driveway of dirt and moss, but the filling material is herausbefoerdert off the rails at the same time, flying in all directions. I was several times so angry, so the Kaercher as already written no longer was used.
The surface cleaner seemed here to be the perfect solution. My expectations were high.
Well ... after repeated use, I must confess, it's not perfect. Heavy dirt or the ugly dark discoloration caused by algae over the years especially in shady areas inevitably, be only partially removed by the cleaning system. It then lacks logically the spray force, so the pressure.
On our ZAPF paving stones in the driveway of the surface cleaner depends unfortunately from almost nothing. Our terrace, which is decorated with anthracite colored granite Patten and bright granite strips, we pretreated before the second wash cycle with an algae remover, which ultimately brought a slightly better result.
One should lead the unit during cleaning without pressure and without prolonged persistence on the surfaces to be cleaned, in other words, the T-Racer is not a scrubbing brush ;-)

Conclusion:
If you already have a high-pressure cleaner from Kärcher, it makes sense, in addition to purchase and try the hard surface cleaner. Personally, I would waive my present experience of both devices to be purchased for the purpose of surface cleaning.