Hi mate, welcome to the forum.
I've built several tamiya kits over the years and my opinion is that they are great fun to build. To answer your question they don't require any soldering, providing you keep the standard electronics and don't change the battery connectors, other than that the kits do simply bolt together. Personally I would recommend the grasshopper as it has a hard ABS plastic body that is easy to paint and doesn't require any trimming which can be difficult for first time builders. The chassis is very easy to put together as well, I've had one for several years now and I've had a lot of fun with i ( take a look at my YouTube channel for some videos, links just below). I would recommend that you buy from modelsport and order a bearing kit with the car as this improves it hugely and is easy to use as it just replaces the plastic ones in the kit. The only thing you need to add to a tamiya kit is a transmitter and servo, the best budget transmitter you can buy is the Absima CR2 SV2 and pretty much any servo will do, if you want something that's adequate then go for plastic gear or if you want something a bit more substantial then go for metal gear, they're more expensive but will last longer and can cope with more stress. All tamiya kits are of high quality and well worth the purchase so whatever you choose you can't go wrong!
Ben.