Interior designer also need to makan. To put in an analogy, it's just like getting a hair cut: You can ask your mom to do it or you can get a professional hair stylist. Obviously the results will be different because of the difference in the breadth of their experience, training and exposure to various ideas.
Unless you are very talented or know exactly how you want your house to look like in your mind, chances are if you design yourself, the house will look ordinary at best.

Budget limitations are always relative and dependent on the condition of the property as well. e.g., older/subsale property with previous occupants may need repair work etc etc.
Definitely doing without ID you will save that cost of several thousands, or about 10%
Hiring ID and asking him to use the cheapest materials is also another way, but I think you will regret that and it may be a lose-lose situation if you are unlucky, i.e., you are not happy with his work and he cannot cari makan. Remember that cheap is usually not good and good is usually not cheap. You have to strike a balance between the two extremes.

If you really want to save money, you could DIY a lot of things and order the raw materials yourself, even fix some of the things yourself if you are so inclined. For example, if you order LED lights, switches or wallpaper online direct from China, you could probably save 50% of the cheapest equivalent local prices, and the quality is not too bad if you do your homework. I know of someone who renovated a 3 storey terrace for under 60K
You can really really save a lot, but really it is a question of time vs opportunity cost. In the example above, that person spent the better part of one and half years doing all the painting, wallpapering and fixing the lighting by themselves, with only the kitchen cabinets and plaster ceiling subbed out to a contractor. If you can afford to spend a similar amount of time doing that you can consider that option.

If you stretch out your renovation period and prioritize the really important stuff, you may be reduce the financial pain of the renovation as well.

Google is your friend The person I told you about did all the research on the net before starting