1. Renovate the back of the house until the back wall about 6 feet ( I know there is a setback but I understand that it is allowed for this taman as many have renovated to the wall). So an additional 120 sqft.

Usually it is allowed, most setback requirements can be easily found through local authority website and doesn't differ much but do not assume that if your neighbour does it, it is allowed.

2. Extend to the side about 12 x 35 both upstairs and downstairs which means an additional 840sft built up

Again, the setback is at play here, depending on the road you are facing, whether it is considered main road or secondary road, the setback distance differs but usually there should be some rooms to add to the side.

3 As part of the extension, the downstairs bathroom will be extended and reconfigured to add an enclosed shower and a japanese style toilet and a laundry room will be added next to the bathroom. The kitchen will also be relocated and new cabinets and an island be built. An L shape bifold sliding door will be added to allow access to the garden from the dining area.

No constraints here except budget.

4. On the second floor, would like to add a bathroom with a bathtub and a large bedroom with a walk in closet. The layout upstairs will be reconfigured with 2 bedrooms and 1 bathroom being extended as well. After renovation the house will have 5 bedrooms 4 bathrooms and 1 laundry room.

Should be able to do it depending on size of rooms.

5. A small driveway allowing me to drive through one gate and exit another at the side of the house.

I could be wrong but I am pretty certain that this is a nono. I am working based on assumption as you did not mention whether your unit is a bungalow or semi-d but in both cases, I doubt this can be achieved. Entry and exit for "taman" residential type typically allocated towards one road only.

My queries:-

a. With so much change in the layout will a budget of RM200K be sufficient?

To be honest, the layout change is possible with that budget but your fitouts i.e. carpentry, sanitary fittings may be insufficient.

b. Do I need an architect?

Yes, it is recommended to use a professional architect but there are alternatives such as runner (experience on this varies from person to person). Anyone who tells you that you can renovate as long as neighbour doesn't complain should be avoided. The price you pay for renovating without endorsement and CCC far exceeds the cost you save from renovating illegally. Just a simple example that you may be fined if your neighbour does complain, and you will ordered to stop work and go through the legal channels to continue. On top of that, i cannot emphasis enough on the importance of obtaining CCC these days.

c. Will such a driveway with another entrance/exit be allowed?

As above

I PM-ed you a few days back with my contact, feel free to contact me if you have any other questions.

how much was your house bought for?

best you seek an interior designer to quote for you first then see where you can chop and save.

I think you could try to get an ID company, ask them all 3 questions. If they able to help to solve on behalf with extra cost then it will save your time and less hassle.

You will know roughly the cost by getting several firm to quote. It will takes time, some wouldn't even reply afterward but it is necessary step.

The budget is really depending on what materials you want to use, location, designs, size etc.

Don't ever assign the job to someone who said they can do it without viewing the unit.

My suggestion,

First, ask local authorities for the requirement docs for approval.

Second, Looking for ID+contractor for site visit. Base on your above-mentioned explanation, you may need architect endorsement and ID design for approval. It's take time for discussion. No need assign any company to lock yourselves. Design/Material Board with quotation will be enough. It will make you more clear understanding about design and cost. Well, you can change your design idea any times during this discussion period. If you happy for that. =P

Third, Assign ID, take the require docs for submission. (eg, endorsement and design if needed.)

Forth, Only Confirm contractor once above-mentioned step done.

Last, Furniture/Fitting move in.
Well, frankly to say that I think RM200k is about the minimum amount for it. As others mentioned , it depend on material and add on. That's the reason I suggested to have a design/material board discussion. 

I already PM you, you can let me know if you have any concern.

Cheers.