A 58-year-old man from rural India presents with progressive dysphagia, early satiety, and weight loss of 8 kg over 3 months. Upper GI endoscopy reveals a large ulcerated lesion in the gastric antrum with irregular borders and surrounding erythema. Biopsy from the lesion edge shows adenocarcinoma. What is the most appropriate next investigation to determine the extent of local invasion and distant metastases?