I am by no means an expert but I have learned a few things.
If you just want to practice movements you can get a solid wood training sword. If you want to spar then there are a number of training options made of rubber or plastic which you can swing around and bang on stuff all day long. Some material is so hard though that you would need to get protective gloves and a mask/helmet.
If you want to practice cutting things I know there are different grades of katana. Probably want to start out with something affordable, like $500 range to go cut the tatami mats and stuff like that. Cold steel has a variety, Hanwei is another well known sword brand that makes useable replicas. Cult of Athena (a retailer) has a lot of sword options. I would say pay attention to the steel because that will probably tell you a good deal. A lot of swords are made of 1055 which is very durable but not as hard.
Traditional katana have soft steel sandwiching a hard steel compared to European swords which tend to be a tempered monosteel. The harder edge of a katana would be more likely to chip than the tempered edge of a european style.
There is no sandwiching. The blade is differentially tempered with the edge forged at higher temperatures than the body/spine, which allows the edge to be sharpened and maintain a cutting edge while preventing the rest of the blade to becoming too brittle and snapping.
European swords (for which I think you are referring to longswords) are tapering spring monosteels, which is essentially spring steel.
A katana has some advantages and some drawbacks. Their curve could make it harder to thrust but at the same time, the wedge shape of a katana blade actually helps to make a cut easier by keeping the blade centered whereas a thinner angle european blade has more chance at deflecting. I think both could hack just fine though, given a sharp edge and similar weight to a comparable sword. I think the slash just improves the strike. Some european swords are curved too like the german messers
This isn't correct. Blade curve evolved over the centuries to accommodate application. Not all katana are hyper curved, with some not being curved at all. It's the same with the kissaki (point of the sword), which went through multiple different configurations to accommodate needs of the time. For instance, the chu-kissaki is the form most people see on modern katana because it provides more structure to the kissaki to resist breaking, at the expense of less penetration potential. Compare that to my preferred style of kissaki, shobu zukuri, which is exponentially. better for penetration at the expense of being significantly more brittle.
With centering, you're referring to what people call self/auto alignment on cuts, which has already been disproven. Blade geometry and weight distribution have more impact on cut alignment than sword type.
The Messer is a solid choice. I would recommend a quality Messer over most katana produced today as it's design is more ergonomic and realistic for combat