Was this a private sale or dealer?
How many miles has it got?
You mention a burning smell..can you desribe that if possible? ie:- rubber smell? oil smell? electrics?
Ok.. To me sounds could be an issue with the gearbox or clutch actuator control module..because you mention thats it seems to be stuck in D4 which means its not shifting down hence the sluggish nature of it pulling off from a standstill and it should be in D4 at 20mph anyway..obviously from here there will be a few things to check out..but you really need this on a high end or Tech2 or OP-COM diagnostic scanner to see what these fault codes are before taking any further steps..so that aswell as gearbox oil condition is a must...Considering its a Dealer sale your still within your rights to return the vehicle.All garages/dealers who operate selling cars are bound by law under the new The Consumer Rights Act 2015 that came in last year - That means you have the right to reject or persue a refund if the car becomes faulty or undrivable within 30days..unless they are willing to repair the defects at no extra cost to you..but you can still reject the car..so the options are there...but as above the diagnostic step from here is the only way forward on this one to see what these codes are..there's not alot you can do a "DIY" bases unless you have the machine to plug in and come back with the fault codes to look into further..
Yep sure..if you can get them with the machine you use...
But if its only been bought yesterday then as above you certainly within the time frame to go down that route of refund..