How much weight are you looking to lose? How realistic do you think your goals are?
I personally haven't experienced big fluctuations in weight, so I cant comment on specific numbers. But I think the general reality is that within a certain range, you can buy the same size, and alterations--if any--will take care of the rest. Maybe that range is 15 pounds, or 20 pounds, but some such range exists so if your goal is within it, then you can pretty safely start buying the clothes you need.
Not exactly sure what you mean by "hobby", and "men's fashion". Do you mean hobby similar to stamp collecting? Because the clothes we wear are an important part of presentation, menswear as a "hobby" is all the in practice--very different from, say, buying Fabergé eggs.
I also notice alot of members have a reaction to the word "fashion." Perhaps it is evocative of the fashion industry, which as a rule is interested in turning a profit, and not so much in making clothes that are comfortable and presentational. Or maybe it brings to mind the metrosexual man (I will be PC here), who thinks loud and garish will "make a statement", and who believes that outlandishness turns heads and is therefore brave (or to use their word, "fierce").
So I'm not sure if that's what you had in mind for "men's fashion", or whether you were referring to the classic code of menswear, which really is an extension of the 19th cen. conception of the gentleman. This way of dressing emphasizes understated elegance, where clothes are both functional and aesthetic; items well cared for will last a lifetime, not just into the next season; clean lines and correct proportions take precedence over ornamental frills; and quality trumps quantity, especially in the people who admire his clothes. The overriding ethos is to convey dignity and gravitas, not tin-eared expressivity or self-centered individuality. So whereas the metrosexual seeks attention by piquing our senses, the gentleman commands our respect by appealing to our harmony and good taste. The metrosexual (and the whole industry built for, by, and of the same group of people) solicits others' affirmation to buttress their own self-worth; the gentleman is already secure in his own being, and naturally coordinates his attire to reflect his inner calm and his immediate surroundings. If something similar to this is what you originally had in mind, then I have a feeling you'll find many helpful things in the answers members post, as well as the articles they link.
It is very easy to over-think matching (I'm assuming you're mostly thinking of color matching, and also pattern matching?). Sure there's probably a science to color coordination, and to mixing lines, grids, and dots, but for a beginner just see if the overall outfit works for you. Get a full length mirror, and check out your ensemble under the lighting you intend to wear it in. If you like it, great; if you don't, fix what needs to be fixed. Luckily, most items in classic menswear go with most other items, so the chance of a jarring mismatch is very low if you stick to the traditional. Experimentation and feedback are key, and hey, the first person you're dressing to impress is you. good luck