Dave is right, pictures or a lot more information would help. I subbed for several appliance companies and have done this many times so I'll try to give you a little help. First many companies make cook tops and built in ovens that are designed as direct, or very little cutting replacements, this would probably be the cheapest in the long run. But if you are determined to put in a stove/oven combo you might consider a slide-in range if your cook top opening is close to the size required by a slide-in range. Using a slide-in range will eliminate having to cut the counter top back splash and removing the cabinet (it can usually be cut for a slide in). But if you are intent on a free-standing range you'll need to remove the cabinet that the cook top is in, assuming that you're going to put the stove in the same location. Flooring or lack there of, can also be an issue when removing a cabinet. Now I have seen cook tops that straddled two cabinets, if this is the case you're looking at lots of work and I would recommend the first option I suggested. A standard free-standing range is 30 inches but you can get them in 20", 24", 36" and 42". The sizes other than 30" are usually more expensive, so if your base cabinet is anything other than a 30" your either going to have to buy an odd sized range, remove another cabinet or use filler strips (probably won't match existing). If your countertop is Corian, granite or anything other than a standard formica you will need special tools to cut it. Assuming that you have an optimal situation, 30" cabinet, cook top cut into only one cabinet, formica counter top, etc., cutting a standard formica countertop can be difficult without removing it (the back splash). You'll have to figure that one out yourself. When you cut a formica counter top you will be left with two exposed ends that can soak up water, swell and look bad. There are iron on end caps, but depending on the age and shape of your counter you may not find a good match, you can use something similar as it should mostly be hidden by the range. Another consideration might be an electric outlet, most new ranges require an outlet for the electronic ignition and depending on how old your old appliances are you may not have one. If you are able to do it your self make sure you terminate the unused gas line for the built-in oven by properly capping it not just shutting off the gas valve if there is one, also replace all flexible gas line connectors, do NOT reuse the old ones. For the empty space left by the built-in oven I have made, TV nooks, Microwave nooks, shelves etc., use your imagination. I don't know your level of experience but gas and electric can be dangerous so call in a pro if your not sure about something, it's not worth the risk. If your up to challenge and have the necessary skills, I hope this helps and good luck to you.
Kevin