Congrats ! What does your dog think of your wheels ? Some thoughts. ON-ROAD vehicles are OK on pavement but NOT good on grass. Some of the most popular multi-mode vehicles out there would be the TRAXXAS STAMPEDE and the TRAXXAS SLASH.
Both are available RTR ( ready to run ) 2WD and 4WD. Since you like tinkering/building they are also available as DIY KITS. I've built both. The build instructions are well written and easy to follow. Because of their popularity alternate and after market bodies are available pre and un painted. K.I.S.S. , ( keep it simple ) but buy a spare battery. Battery ? Since the dog walks will be liesurely, BRUSHED motors and NiMH batteries with a TRAXXAS ID changer will do. As you grow and SPEED becomes important, BRUSHLESS motors and LiPO batteries will become desirable. The higher milliamp-hour rating your battery has the longer it will run before recharging is needed. TRAXXAS transmitters also have options for BEGINNER and ADVANCED modes. BEGINNER might make a leisurely perambulation with your pup more pleasurable

You also might like to do some video filming on the cheap. Micro-size cams ( dashcams ) can be had for well under $50 through places like WALMART. Have fun, enjoy; you never know where this great hobby will take you; FPV ? TELEMETRY? BOATS ? PLANES ? QUADROTORE ? ... ... ...