Pictures would help as context is key. Would you be hiring out or DIY? Do you have a tub in another bath? Who lives here? Any kids? If not any planned? Help us help you.

Do you have bedrooms on the main floor ? If not no need for a full bath IMO but we need a lot more info, IMO both kitchens and bathrooms should be up to date as far as use is concerened no matter how old the house we have different needs in 2023 that they had in 1905 for instance.

One tub is ideal in a house as some home buyers won't consider a house without one (when we were looking for houses I required it bc I had babies/toddlers to bathe) - but if you're planning on being in this house for a long time then do what works best for you and your situation.

If it were me, I would start with a plumbing estimate from a pro who has lots of experience/patience with older homes - if you're tearing into things it might be the ideal time to replace/upgrade pipes - they often can be very creative problem solvers (I worship the plumbing team we work with lol)

Usually, families with kids like to have at least one tub. Probably not in the master bedroom. A house with no tub could reduce your resale value, but this will depend on location. If you are near a great school district or other area attractive to families with kids, it might be a problem. If your house could be marketed as good for seniors, a shower big enough to put a stool in might be a plus instead.

We added a half bath reconfiguring some closet soace. we just had contractor out to confirm feasibility and cost. we made design ourself though and didn’t use architect or designer. definitely keep one bath. shower bath for kids is fine.

Unless you have exceptional DIY skills, you need a GC. My husband is pretty good, but this is a bigger job than he would undertake. One of the advantages of a GC is that there are some great plumbers and other subs who will only work for GCs. Some of them aren't interested in working directly with homeowners. Also, if you are your own GC you have to know exactly what you want done, the order it needs to done in, and schedule everything yourself. Plus pull permits, schedule inspections, etc. If you have a day job it's just too much IMO. If you have the budget for it, also engage a NKBA certified bath designer for the best end result.

It sounds like you need to start looking for some general contractors to look at and bid the job for you. They should be able to let you know what can be done and give you a general idea of cost. Someone will need to evaluate structural issues (moving walls, supporting bathtubs, etc) and routing of plumbing. I do think it's nice to have at least one bath tub in the house. Whether it's for bathing kids or dogs or soaking something, it is nice to have. I do think a really small place can sometimes get away without (such as a 2 bedroom home). Keep in mind that a bathroom without a tub is a 3/4 bath. A bathroom with a tub and shower is a full bath. That can matter for resale and value.

You need a design plan from an interior designer who specializes in kitchens and baths to help you draw up the proposed layouts. Then you can have a contractor bid the job. Having a plan will make the bid far more accurate. And if you have multiple bids, they'll all be working from the same plan so it will be a better comparison for you.

Whether or not you can do this will depend on how much space you have. Bathrooms have code minimum spacings. Start with a measured diagram of the whole space, with windows, doors, plumbing locations, etc. Indicate the rooms around it, including closets, halls, and bedrooms. That is where you start space planning to see if this is even possible. After the design work has produced a concept, then you can talk to contractors about costs. The more you move, and alter, the more expensive this will be. https://www.remodeling.hw.net/cost-vs-value/bathroom-addition?y=2023